I am desperately in need of help! :-[
I own a TR1 -81 and have the following problem.

When braking or going over a speed bump without a bit of throttle the bike wants to stall.
Whenever I do semi hard brakes the engine revs down and if I give it too much gas just after stopping the engine backfires and dies, sometimes so violently that the carb jumps out of the the intake manifold. What I usually have to do is jerk the gas two or three times quickly. On the fist jerk, you can hear the engine "struggle" and after the second jerk the engine revs and sounds like normal. I've have pulled the carbs many times in order to set the float level but without success. Every time I pull the carbs there is a little bit of fuel in the intake manifold on the air side of the carb to the rear cylinder.
Apart from the braking and speed bump issue, the bike runs fine. There is some occasional bakfiring when decelerating  and a little bit of fouling of the rear spark plug. Furthermore, if the bike sits for some time and I start it up and shut it down immediately it will only run on one cylinder when I start it up again due to a fouled rear plug.

I have replaced all vacuum hoses and fuel hoses, the carb has been cleaned using ultra sound, I have rebuilt the carb(jets, needles, pilot screws, etc).
After searching the Internet for quite some time I have narrowed it down to incorrect float level in the rear cylinder carb. I have tried to adjust this level without any real success. I have been pretty careful when adjusting the floats and have only done minor adjustments to the tang (maybe too careful, meaning no real adjustment). I did make sure to set the level according to the specs in the Haynes manual, by measuring from the top of the float to the mating surface(at least I think I did).

What do you guys think, is it float problem or something else?
To prevent the bike from dying whenever I brake I set the idle to approximately 1500rpm. This helped a bit but still, the bike wants to die during medium braking and dies completely during hard braking(panic braking).
I have synced the carbs and set the mixture screws with colortune.

Hi Bergan,
Did you ever check or replace the carburator membrane? Does the slide of both carbs revert with the same speed, if you move it up by hand?

Hallo  Benghan,
Have you searched for air leaks, are the intake plugs tightly attached?

No, I haven't replaced the membrane and I have tried to move them by hand. The both feel the same. I did also think suspect the membranes a while back but after moving them with my hands I ruled them out. But now that you mention it I might replace them. Are busted membranes known to cause the problems I describe? If I'm not mistaken I did notice something that looked like a small bubble och dent in one of the membranes, approximately 0,5mm in diameter. But since they felt the same when moving them by hand I ruled it out.
But let's say that I have a small hole in the membranes. Could this cause fuel to overflow the carb when braking and also be the cause of fuel in the intake manifold on the air side of the carb?

I have searched for air least a couple of times with starter gas, but no leaks.

I have never ever heard of brakes effecting a carburator. :D maybe if we'd talk about cars with powered brakes, but not at a bike!
So it has to be the change of the air flow, the change of RPM or the fuel in the carburator that moves while braking. Therefore I thought about the membrane...
Or the needles might not seal the Jets, so the fuel can pass them while breaking? But i've also never heard about this  ;)
maybe the needle of the floats does not seal properly? To mention another idea...
The fuel within the gas tank also moves while breaking. So maybe this could effect the pressure to the floats.

I never stated that the brakes were affecting the carburetor, rather that braking affectes the carburetor ;D
I will tear the carbs down during the weekend and go over all the seals and o-rings. Did a complete rebuild when I bought the bike 5years ago. I will take a closer look at the membranes, float level and needles and get back to you with an update. Thank you for the feedback!! Please let me know if you have any other ideas as to what might be the cause of my problem!

It is hard to say what the reason is, since you have done everything. I would try  to remove the vacuum hoses from the petcock, close them and go for a ride with the cocks on PRI,
may be it helps, may be not.  What you describe sounds like a much too  rich mixture at idle,
Regards Ali

Ali wrote:

It is hard to say what the reason is, since you have done everything. I would try  to remove the vacuum hoses from the petcock, close them and go for a ride with the cocks on PRI, may be it helps, may be not.  What you describe sounds like a much too  rich mixture at idle, Regards Ali

I tend to Ali's theory, via a defective membrane in the petcocks can gasoline into the intake port. This error I had already.

werner r. wrote:

[quote author=Ali link=1490602287/0#6 date=1490682318]It is hard to say what the reason is, since you have done everything. I would try  to remove the vacuum hoses from the petcock, close them and go for a ride with the cocks on PRI, may be it helps, may be not.  What you describe sounds like a much too  rich mixture at idle, Regards Ali

I tend to Ali's theory, via a defective membrane in the petcocks can gasoline into the intake port. This error I had already.[/quote]

Thank you for the replies! I replaced the petcock membranes last summer so they should be fine. I will definitely try your method anyway just to be sure Ali.
Any thoughts on measuring the fuel level in the bowls? Can this be done with the transparent hose method with the carbs on the bench or must this carried out with the carbs mounted on the bike and the engine running?

I received the last spare parts today after a long wait and because of that i wasn't able to take apart the carb before, so no status update.
What i have ordered is a set of new diaphragms(ebay) and new intake manifolds(original Yamaha) from my local bike shop.
Before i placed an order for the manifolds I went to my local bike shop to buy som o-rings for my old inlet manifolds. To get the correct dimensions we looked at the microfilm the shop had. There was no sing of the o-rings just a gasket. I told him that my manifolds have a small groove for the o-ring and therefore there must be an o-ring. But i could clearly see that there was just a gasket in the microfilm. I know that my manifolds are in bad shape so i thought that i should order a new pair of original manifolds while I was at it(i have tried the ones available at ebay but don't like them). All said and done, my parts arrived today and upon unboxing i noticed that the manifold kit included the o-rings. And sure enough there was a groove in the manifold for the o-ring. My question is this, since the exploded view in the microfilm clearly stated that there should be a gasket present should i use both the gasket and the o-ring? Or should i use just the gasket or just the o-ring?

As far as I know, the old manifolds had a gasket and should be used with sealing compound to be free from leakage. Therefore the next generation of manifolds was designed with an O-ring.
The new manifolds i have installed 4 weeks ago have the o-ring only and it works great. I wouldn't use both.

Yes, originally there was a paper gasket and the newer ones have rubber o-rings. I once had an o-ring that was missing partly after several years. I believe it was a little bit thin and could move within the notch and vaporised itself over the years. I replaced the ring with a slightly thicker one.

In the meantime i have had a piston jamming on my RD500, the intake manifolds where leaky just at this o-ring (which are solid connected to the manifold there). So menawhile i recommend to mount every intake manifold, whatever kind of seal it has, with some silikone sealing compund, such as Dirko or Hylomar.

So yesterday i removed the carb and took it apart. I tried to remove the diaphragms carefully, but without success. since it has been raining here for the last moth and because the forecast promised sun today i did not have the guts to risk screwing thing up(would have been the first ride of the season). However, i noticed that the diaphragm to the rear cylinder carb had a small area that was brown (approx 1mm i diameter) on the "piston side" and a bubble in the rubber (same size) in the same location but on the "top side". I used some rubber cement (small amount) to patch it up. Before i removed the carb from the bike i checked the fuel level in the bowls and noticed that the fuel level in the rear carb was a bit low, approximately 4mm below the mating surface. So i checked the float height with a digital caliper set to 15.8mm and one of the halves of the float was indeed touching the caliper. So i readjusted the float and put the carb back together and mounted it on the bike. I checked the fuel level and the level was pretty much spot on. The mating surface was almost perfectly in between the top and bottom of the U-shape in the tube (due to surface tension), so i guess the level is ok?
I started the bike and it sputtered much so i grabbed my colortune set and started with adjusting the rear cylinder. The mixture was rich so i adjusted the pilot screw so that the flame was blue. At this point the bike would idle better and the sputtering had stopped. So i moved on to the front cylinder and i could hardly see anything else besides the spark, i.e no flame or a very weak one, not even when pulling the throttle. I went for a ride and there was no power in the bike and a very muffled sound from the exhaust compared to before. During this process i replaced the manifolds.

My questions are the following

1. what is the best way to replace the diaphragms? i have read that you should cut away the metal ring on the top and then use superglue to hold the diaphragm in place. Another method i have read about is one where you use a small screwdriver to push the plastic ring down in order to pull the old diaphragm out. The last method is to just tear the old out and use dental floss to coax the new in :-/.

2.could the muffled sound, and engine running on one cylinder(spark+no flame=no fuel?), be due to a vacuum leak or bad sealing between manifold and mating surface? i had a bit of trouble mounting the carb on the bike due to the new manifold not being as soft and worn out as my old ones and in all this frustration i might have forgotten to connect the vacuum line to the petcock on the right side of the fuel tank ::) (embarrassing if so). This could explain the lack of flame/fuel. The bike is currently at my parents house so will check this tomorrow on my way home from work.

3. upon removing the piston/diaphragm assembly i noticed a small amount of black particles in the cavity where the choke tube goes(under the diaphragms seen from the top). Is this coming from the small filters and if so can these be removed without any impact on the engine? if not, do you guys know of any suitable replacements?

I read this and am still unsure how the carbs might be affected by braking, certainly it sounds as if you have carb issues and the new manifolds are a good idea but I am not convinced that the engine stalling issue when braking has to do with that..

What condition are your plug leads in? How good are the connections between the coils and the wiring harness? Check both the power and earth connections between battery and bike for anything that might move when braking..

I had a similar issue with intermittent power loss on my XV750SE and it turned out to be a (partially melted) cable that got too close to the rear header and would touch when braking/speedhumps and cause the engine to stumble. I don't remember if it went to the reg/rect or the generator but it was a single thin black cable.

I also found an issue when rebuilding where the ends of my plug leads had corroded (wire gone green) and I cut them back to clean wire which helped and last but not least I had a loose connection in one of the coil connectors (orange/white) that caused intermittent running on one cylinder..

I have had two different sets of carbs on my 750 (one bought as spare was cleaner looking so I swapped it out) but I have never had them apart, only used fuel additive cleaner and sprayed carb cleaner through them with the engine running.. Every time I read another tread of people opening the carbs up and having trouble I wonder why I never have, I cannot have been just that lucky with two sets of carbs?! Granted, someone who knows  they are doing could probably get my bike running stronger but I don't, never been inside a carb so I opt for leave well alone...

I recently did my valve clearances and was amazed at how much my rear cylinder was out (almost seemed like it was set at the wrong timing mark) yet the engine still ran, on both cylinders..! Just to say, these engines want to run, I hope you find your issue soon! :)

Cheers,

Rant.

[full quote deleted by moderator]

The reason why i suspect that the problem when braking is originating from the carbs is because i find raw, unburnt, fuel in the air intake to the rear cylinder carb each time i disassemble the carb which to me indicates an overflowing carb. This is my first motorcycle and my thus my first experience tinkering with one. My conclusions are therefore based mainly on what i read on different forums. In all my reading i have not found anyone with a similar problem :( With this said i welcome all suggestions and possible reasons to why i have this problem.

Another reason to why i suspect the carbs is that the guy that owned the bike before me did was not much of a mechanic. I bought the bike i "working" condition and since i did not have any license or had even driven a motorcycle before i had no way to test the bike other than to trust the seller and watch him test drive it. Once i had the bike at my place i went through all the hoses and connections and found that the previous owner had been running the bike without the vacuum hose to the rear cylinder, just to name one example. Who knows what else he has had his fingers on :o

A little update: It turns out that i did indeed forget to connect the vacuum hose to the front cylinder petcock. After reconnecting it i synched the carbs and adjusted the pilot screws and it turns out that the new manifold did help a lot. The bike runs smoothly throughout the rpms and there is no sputtering. Before i had to set the idle at 1500 to prevent the bike from sputtering when releasing the throttle. Now it runs "flawlessly" on 1100-1200 according to Haynes. However the problem that occurs during heavy braking still remains. I will take a lok at wires as you suggested Rant and get back to you guys. I just had another wisdom tooth removed today and i am currently on citodon so both me and the bike will have to rest for a couple of days.

Hi Benghan

Good and bad information from my side.

Good: you are not the only one who has this problem
Bad: I never found the reason why this problem occurs when braking hard. I haven’t been searching very long, but I was trying most of all the suggested advices. In vain.
Fortunately, I’ve got then some used carburettors, with these the phenomenon disappeared. I always planned to search more in detail for the reason but never had the time....
So at least on my bike, the carburettor was the problem!

Greetings
Edzard

Hello everyone!

Some time has passed since i last posted here and i have had som time to keep on figuring out the cause of my problem. It pains me to say that i am still at square one. Yesterday i replaced the carbs with a pair i bought from ebay without any success. Unfortunately the drain plugs to the bowls of the new pair were damaged so to speed up the process i took my old bowl and mounted them on the new carb. To completely rule out the carbs i will replace the old bowls with the new ones this week when i have removed the stuck drain plugs ( which are really stuck with destroyed heads as a bonus ).

One question arose when i cleaned the new carbs: There is a small hole in the lower parts of the bowl which at first glance seems to be connected through a long drilled hole to the port and the small brass tube attached to the carb ( i think its the overflow tube). The thin brass tube on the carb is inserted in the long drilled hole when mating the two parts. To my surprise there seems to be a brass plug in the drilled hole which cuts it of from the small hole in the bottom of the bowl. Is this supposed to be plugged and if so, what would happen if this plug is removed for some reason ( by previous owner).
One of the replies in this thread mentioned replacing the carbs to solve this problem, and since i have replaced the carbs except the bowls i want to know if this hole should be plugged or not.

The hole and the brass tube are for the choke. The "plug" in the hole is originally a jet, but it is often clogged and so works as a plug...

Are you sure that this it is a jet? I went in with a borescope today at work after a few hours in the ultrasonic cleaner and there was only a small indentation in the center of the plug probably made by me poking around with a thin steel wire trying to get the worst dirt out before the ultrasonic cleaning. If this is indeed a jet, do you know the diameter of the hole because i tried sticking in a 0,7mm wire while looking down with the borescope and the wire diameter was at least twice that of the hole.

In the attached picture:
#1 Green arrow: What is the function of this hole? it is above the fuel level in the carb so i guess its not for sucking in fuel to the choke.
#2 Is there a free passage in to air/fuel flow (main flow in to the cylinder) through the brass pipe? If too much gasoline is forced in through the green hole during braking could it overflow into the "main flow" given that the hole is plugged?
#3 If the hole is clogged, wouldn't this mean that the suction end brass pipe is constantly under the fuel level? Is there only a suction of fuel/flow of fuel through the brass pipe when the choke plunger is in an open position?

Many questions at once... ::)

Hello

Where are you based?

The jet sits in the float bowl, in the hole where the brass tube is inserted into.

I am based in the southern parts of Sweden.

The most recent update follows.
A few weeks ago took a second look at the bowls and you were right, the "plugs" are jets. Turns out that the ultrasonic cleaner cleaned everything except the jets... I ended up pushing a 0.2mm drill bit through the dirt! I decided to take a look at the floats since i have been convinced that my problem was caused by overflowing carbs so i followed the haynes manual thoroughly. The measured fuel level was, according to manuals and instructions on other sites, too low made no sense to me since i was convinced it was the other way around. I set the floats according to the manuals without any success. The next thing i did was to cut the fuel supply, let the bike idle and rocking it back and forth to see if there was a point were the problem disappeared. The bike died a few times but after about 1-2 minutes i could rock the bike violently without it dying. So i removed the carbs once again, set the floats slightly lower than specs, and in a way so that the floats weren't parallel the fuel level upon closing of the needle valve and mounted the carbs again. I fired the bike up not expecting any difference tried a medium brake and nothing, it didn't die on me. so i started rocking the bike and it jest kept on purring.
In short terms, i have finally solved my problem which did only take a few years to solve....  ;D I haven't had time to check the actual fuel level again after i got it working but will do so when i have the opportunity. Could the problem have been caused by the floats closing at somewhat correct level but not pushing the needle enough so that upon braking the the fuel level shifted causing the floats to temporarily let of the pressure on the needle causing excess fuel to flow into the bowl?

Thank you everyone for your comments and ideas. To anyone who experiences the same or similar problem/s as i did, my recommendation is to cut the fuel supply and let the bike idle to see if the fuel level in the bowls is the cause.

After sitting for 19 Years, my TR is in need of some attention. 1st order new ignition coils. 2nd order new inlet maanifolds.3. replace some of the vacuum hoses that were a bit too big to be snug. 4. clean the tar out of the carbs & jets..
VAST improvement.! When the new inlet manifolds arrived (no O rings) I headed off to Seal House to find some. I couldnt find any reference to size anywhere in the forum, so was hoping the shop might help.
The little bloke told me he couldnt get anything bigger than a 2.0mm to fit because there were little castings in the O ring race.
He suggested that Yamaha cocked up and I should remove the offending bumps so as I could get a bigger O ring to REALLY seal that gap.
New to this, I consulted my mate the engineer (German trained; BM's all through Europe etc. Worth knowing) Consequently...
Gasket would def leak! (1981)
Too big of an O ring is BAD-.Too much crush.
As the inlet is working in VACUUM, the O ring needs to be snugged to the inner edge. (pressure system = outer edge)
Castings in the race hold the O ring to the inner edge.
Race on this manifold was 1.49mm deep ~ 2mm O ring. Inner edge diameter was 44mm. I have ordered

2 of  44 x 2 N70 o rings ( for those looking for an O ring size)
Hope this helps