Chaising the infamous hanging idle - Carb tops possibly?

The more you change out these engines the faster and easier it gets.......

I had a leak on this rebuilt '83's engine. Right side, top of the jugs. It was time for a retorque anyway and I had initially torqued the main studs at 30 Ftlbs. Going through my notes I saw that stock torque was for 23 or 24...... Whatever it was that's what I retorqued to. Forty miles on so far and no leak. Of course it could have been that 12mm headed bolt to the left rear that somehow got missed the first time........lol

5twins was mentioning a spray on gasket sealer that he uses for the head gasket........copper something.
 
Yeah, I believe i used the copper spray when i assembled it originally. because the leak is enough to affect the fuel mix, I'm starting to think it might be warped. But, the only way to tell for sure is to go through the hassle of another tear down unfortunately. Such is my luck.
 
Sci, I have some doubts about your diagnosis. If the head gasket was leaking so badly that it affected the fuel/air mix, it would also be pumping oil in a very big way, and pressing down on a carb top would have no effect. Are you 100% sure that there's no leak between carb boots and cylinder head?
 
Are the carbs all the way into the boots ? did you adjust the Idle air screws ?
....
Bob.........
 
I do have brass. I fiddled with it again this morning and found that the right cylinder is misfiring. The rechecked the carb sync and they were good. But I did notice that if I turned the sync screw so they were out of sync, the high idle went away....

It goes away adjusting the synch screw? But the carb synch is good?
If cylinder to cylinder compression is within parameters, if the exhaust pulse is the same after you readjust the 'good' synch then it seems to be cured.
 
It goes away adjusting the synch screw? But the carb synch is good?
That's why I was suspecting misaligned shaft or worn out body or bad seal; just poking the shaft could have an effect then.
I'm with Griz on this. On the other hand, spraying the head gasket appears to show it's leaking around the gasket...
 
xjwmx - I did indeed try pushing on the shaft end. No effect. I sprayed the shaft ends after installing the new seals. No change.

Are you 100% sure that there's no leak between carb boots and cylinder head?

I'm almost 100% positive. I've sprayed around the boots multiple times thinking the same thing and there's no change in the idle. I know it's crazy no?

Are the carbs all the way into the boots ? did you adjust the Idle air screws ?

Yes. Yes. I've pulled and reinstalled the carbs so many times I can do it with my eyes closed. Right now, the fuel mix screws don't have much affect due to the leak. I've also tried 45's and 47.5 pilots. Right now, I have the 45's in.

It goes away adjusting the synch screw? But the carb synch is good?
If cylinder to cylinder compression is within parameters, if the exhaust pulse is the same after you readjust the 'good' synch then it seems to be cured.

The high idle drops back to normal when I "adjust" the sync screw. I started with the carbs synced using the manometer. However, it idles high in that case. If I back the screw out so they are now NOT in sync, the idle comes down. So it appears one side is having the issue. The right side exhaust pulse is weaker then the left and misses periodically. Cold compression with no oil squirted in the cylinder is 110 left, 100 right which seems a little low. I will take a new reading with oil squirted in the cylinder and see what I get.

I know this is such a bizarre problem. But what I do know is that spraying at the intake side of the head gasket definitely drops the idle every time.

At this point, I just don't know what else to do...
 
Here's a video of what I'm talking about. First is pushing the right carb down which I think is just manipulating the butterfly's. The second is spraying at the head gasket. Took me a few tries to get the aim right haha. Then, the last part is adjusting the Sync screw so they are OUT of sync. Note that when they are out of sync, I can blip the throttle without it high idling but then, like I stated, the right side cylinder doesn't run as well and misses some.

 
If your mix screw settings are mismatched, that can cause the idle to hang. The BS34s are so E.P.A. stifled, their mix screws don't have as much of an effect as the ones on the older carb sets do. On the older sets, I can kneel next to one side/cylinder, adjust it's mix screw, and easily hear the idle speed drop off and stumble if I go either way past the "sweet spot". That "sweet spot" is a pretty narrow range too, about 1/2 turn at most. The BS34s don't behave this way. There really is no narrow "sweet spot" that easily shows. You practically have to close the mix screws completely for the idle speed to drop off. It's very difficult to do one cylinder/mix screw at a time so I use a different technique.

I set both screws/cylinders together and rely on small throttle blips to judge the setting. I start with both screws matched and set at, say, 2.5 turns out. That's usually too lean and produces popping and idle speed hanging when doing the little throttle blips. Then I start opening them up, together and the same amount so they stay matched - 2 3/4, 3, 3 1/4 turns, etc. Eventually the popping and hanging idle will go away, usually in the 3 to 3.5 turns out range somewhere. But as I said, it's important to keep the settings matched between the carbs, or at least very close.
 
Test it motor warm, throttle held wide open, you'll get better results.
 
Test it motor warm, throttle held wide open, you'll get better results.

I will indeed.

At this point, not sure what other tests I can do. After seeing the above video, you think it's the head gasket or carb related?
 
I don't think it's head gasket related. Like Griz said, if that gasket was sucking in air, it'd be belching out oil big time too. How old are your manifolds? And if new, what brand? The way you solve the issue by pressing down on the carbs seems to indicate the manifolds aren't sealing until you do push the carbs down. Are the clamps as tight as they'll go? They need to be tightened all the way, until the two clamp halves touch. Also, many new manifolds come with just a thin paper gasket. The originals are very thick, a good 1/8" or so. I think you need that. I won't even use those thin paper ones.
 
To eliminate the head gasket once and for all, put a straw on the can...I notice there's no straw...then direct a small spray right at the gasket, just a little bit at a time. I can see how some bit of gasket could blow out, burn out, etc. out, and not make it rain oil. Theoretically speaking. The way to attack this is to eliminate each possible cause one by one, not figure out what the cause is.
 
I might be inclined to put a gasket sealer on the manifold to head gasket just to eliminate that possibility, thank goodness for paint stripper when it's time to clean that off........also like 5twins said, screw them clamps down tight.
 
Ok, to answer some of the questions:

These are brand new manifolds. I don't recall exactly where I got them but probably Mikes or 650central. They are nice and soft.

Clamps fully tightened. And I have sprayed all around the boots and the manifold. I also ditched those thin gaskets and made up a new set using thicker material that I've used before and works well. I then sealed them just to make sure like someone mentioned.

I think pressing down is just moving the butterfly. Notice towards the end of the vid when I adjust the sync screw. I'm turning it out which is closing the right side carb's butterfly valve and the engine then idles down.

Believe me, I'm just as perplexed as you guys. I will try the straw and very precisely spray the head gasket and boots and try and get a video of it. But, everything so far is telling me that head gasket is mucked up. Thanks for all the input guys. I really appreciate the help...
 
Personally I think 5Twins answer above is the answer, adjust the airscrews ! or do a "dead cylinder adjustment"...
I had trouble with mine till I did that and it cured the problem.
f.w.i.w.
Bob......
 
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