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Another tale of carburetion woe

Posted: Sun Nov 02, 2014 2:44 pm
by desertrefugee
Well, I've had the opportunity to put some miles on my recently re-streeted '78. Just under 400 miles in a week! It has been running well - mostly. I've had an off-idle stumble since getting it back together. During my refurb, I had the carbs off no less than three times - on the first giving them a complete rebuild (not the Randakk kit).

What I now notice is intermittent behavior. Besides the off-idle issue, the most noticeable issue is that it will run like dog doo just after coming off the freeway. Did this twice yesterday during a 200 mile day. Each time, it cleared up in a couple of minutes.

This morning, I wanted to blow more cobwebs out and took it out for a solo run. Ran like dog doo pretty much the entire time. I turned back home and threw the IR gun on it. #2 was running about 15*F hotter, so I ran the idle air screw out one full turn. I don't know it it was coincidence or what, but I took it for another spin and it was running well again.

So, I'm sure I have idle circuit issues with regard to the off-idle issue. But what gives with the changing behavior. Sticking float? Slide hanging up?

I had actually intended to pack these things up and send them off to Pistol Pete. I even spoke with him. But, things got better and I held out.

Now, I'm thinking I'll send them off anyway. But, that's a pretty pricey (for me) decision...

Re: Another tale of carburetion woe

Posted: Sun Nov 02, 2014 3:49 pm
by chewy999
desertrefugee wrote:I had actually intended to pack these things up and send them off to Pistol Pete. I even spoke with him. But, things got better and I held out.

Now, I'm thinking I'll send them off anyway. But, that's a pretty pricey (for me) decision...
I was in a similar situation when I started. The bike was only firing on three cylinders, so, long story short, and after loads of help here, I stripped the carbs down myself, and cleaned them with carb cleaner and reassembled using the same seals and it worked to a point. Contacted Pistol Pete, and was ready to send them, but in the end used Old Fogey in Scotland to save postage. (Pistol Pete and Moose were most gracious when I explained my decision to them). Anyway, think you will find if you get your carbs built properly using Randakks kit, you will realise it is actually money well spent.

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Re: Another tale of carburetion woe

Posted: Sun Nov 02, 2014 6:55 pm
by ericheath
How's the inside of the tank look? Check the upper sides with a mirror.

Re: Another tale of carburetion woe

Posted: Sun Nov 02, 2014 7:24 pm
by zman
It is not impossible that you could have a piece of an old oring stuck in the idle circuit passageway, with rubber it would be difficult to clear out by riding or additives, and I think there are 3 holes in the throttle bore that should all be clear. I`m going on the assumption you did everything else correctly, and the blockage could have happened well before you bought the bike..

Re: Another tale of carburetion woe

Posted: Sun Nov 02, 2014 11:27 pm
by roachb
If you find the cause please let us know because I am chasing a similar issue.

Re: Another tale of carburetion woe

Posted: Tue Nov 04, 2014 2:02 am
by jdvorchak
I'm a little fuzzy on the whole "running like dog doo" description...

I've been working on cars, trucks and just about anything with an internal combustion engine for about 50 years. I've hear of "cutting out" "dead miss" "won't get above idle" "idles too high or too low" but "dog doo".. Must have missed that one...

Is it dropping a cylinder or two?
Is there poor throttle response because it's too lean or too rich?
Did you remove the air dam behind the radiator and now your carbs are getting hot and semi vapor locking? The fact that it clears itself after a couple of minutes seems to indicate that it is a heat related issue.

Re: Another tale of carburetion woe

Posted: Mon Nov 24, 2014 3:38 pm
by desertrefugee
jdvorchak wrote:I'm a little fuzzy on the whole "running like dog doo" description...

I've been working on cars, trucks and just about anything with an internal combustion engine for about 50 years. I've hear of "cutting out" "dead miss" "won't get above idle" "idles too high or too low" but "dog doo".. Must have missed that one...

Is it dropping a cylinder or two?
Is there poor throttle response because it's too lean or too rich?
Did you remove the air dam behind the radiator and now your carbs are getting hot and semi vapor locking? The fact that it clears itself after a couple of minutes seems to indicate that it is a heat related issue.
I guess what you must have missed was an attempt at brevity and/or humor. (look it up).

A few more details, beyond what I offered up, may or may not have helped someone put their finger on my problem. I guess I could have added that it has been running a bit lean in all four holes. All I was looking for was ideas. Thanks for providing yours.

I've seen the inside of carburetors ranging from SU to Amal to Carter/Edelbrock to Holley and have yet to run into the kinds of issues I have on this set. I suppose the jetting could simply be wrong, but I don't believe they were ever touched before I got into them. Nor do I believe the Sportster mufflers are the culprit. While I agree that the off-highway symptom suggests a heat-related issue, the air dam is very much in place - and the low RPM stumble is a constant.

Anyway, it really doesn't matter now as I've thrown in the towel and sent them off to Pistol Pete. I've promised them a Christmas card if they can also let me know what they might find going on.

Hats off to those who've been successful with their rebuilds. I have neither time nor patience (or apparently, ability) to get to the bottom of what ails this set.

Re: Another tale of carburetion woe

Posted: Mon Nov 24, 2014 5:10 pm
by chewy999
desertrefugee wrote:I have neither time nor patience (or apparently, ability) to get to the bottom of what ails this set.
I actually found working on the carbs interesting and relaxing. Considering it was the first time I had attempted carbs, it was quite successful. Used the original seals and it worked better than when I started. That said, I did end up sending them to Old Fogey to get them done properly. But there are good guides in shoptalk and if you take your time and do each carb separately, it's good fun. Definitely do it myself if needed in the future.

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Re: Another tale of carburetion woe

Posted: Mon Nov 24, 2014 6:15 pm
by rcmatt007
78's don't have an off idle issue, that was something on the earlier wings. So if you have one, the carbs are not working right at all. Slide problems are not when you open it up (or shut it down) as the vacuum overcomes the stickiness (and if they are stuck that bad it won't really run). Sticking slides will give you a little running rough at a steady speed.