Trying to make my 430 into an 860 again.

[JD]

Ive been working on my 1975 860gt, more like fighting with it. The horizontal cylinder has been intermittently firing. I've checked spark, and have plenty of it. Even stronger through the advance. Whats happening is, or I think, the carb isnt getting enough fuel. First, I dont see the fuel moving through the lines like I do the rear carb. Second, when the intake air boot is removed, I can completely cover the mouth of the carb with the bike still running.then, the fuel starts moving through the line. When I release my hand, the cylinder starts firing very nicely for about 3 seconds. I've tried running the bike on just the front carb. I give it a few squirts with the throttle and kick it. Again, it only runs for a second or two. The carb bowl is full, as far as I can tell when I remove it. Ive played with the fuel taps, and fuel will pour freely from the needle valve when left open. Float level off? The plug is new and is not hot when the bike has been run. The cables are brand new and adjusted with the slightest bit of slack. Both mix screws are out 1 1/2 or so turns. Slide screws have been adjusted in and out. The manifold gaskets are new, not cracked. The carb has been removed and all jets have been cleaned with carb cleaner and blasted with air, as have the air passages. No changes. The carb should at least run badly. Right? Thanks in advance for any help.
JD

[nottonight68]

you have'nt had the heads off have you?

[JD]

I have just recently bought the bike. It has 8700 original miles. This bike has been untouched. Stunning original condition. Still has the original safety wires on the exhaust flanges on both heads and on the cases. Carbs were rumored to have been rebuilt fairly recently. How well, dont know. Thanks.
JD

[nottonight68]

check the operation of the chokes

[ozihotrod]

sounds like an air leak or blocked carb, may be a blocked drilling in it, can the carbs be swaped from one cylinder to the other to seen if that makes a differance, have u check for compression, could be a vavle not fully seated

[Lumpy]

Quick easy check is the filter screens on the end of your fuel taps. They stick up into your tank and filter the fuel coming through. My GTS turned into a 430 on a lonesome highway once and I pulled over and noticed no fuel movement so pulled off the fuel line, blew hard through it and that got me going again till I got home where I cleaned out the tank which was full of rust, scum and who knows what. For a quick check pull off the fuel line and blow compressed air through it (make sure your tank filler cap is open and not filled to the brim with fuel)and try again. If it works, you`ll have to take the tank off and clean it out. I put one of those motor cycle tank treatments through mine. Takes about 3 days for the stuff to go off and cost about $75. Let me know how you go.

[abmartin]

I've never found that watching fuel move through the lines is any indication that the carb isn't being supplied. Very little seems to run through the lines, particularly to the front cylinder yet my bike still runs. The fact that there is fuel in the float bowl shows that everything is ok in that department.

What happens when you rev the bike harder? Does the cylinder pick up then? Have you tried actually riding it? You said earlier that it pulled well before the throttle cable broke. What has changed besides fitting a new cable?

I have found that its not uncommon for the front cylinder to be a little lazy at idle. Sometimes it clears itself up after riding for awhile. Often revving the bike will help. As well, make absolutely certain that the throttle slides are lifting simultaneously. You may also want to check the tappet clearances - easy to do on these bikes. Another thought, although it shouldn't affect the carburation at lower revs is to make sure the needles are in the correct position. Finally, although you say the bike is unmolested, I have seen machines where the bevel gears at the cylinder head are not correctly timed.

My guess, though, is that it is a simple carb adjustment problem or just lack of use.

Bruce

[abmartin]

One more thought - make sure the carb is aligned vertically.

As yours is a 1975 you should pull the rocker arms to check the chrome plating on the portion of the rocker arms that bears against the cam. Both my friend and I had '75 860GTs and we found that peeling and checking of the plating was a real problem area. We did a lot of mixing and matching before we found good ones.

Its easy to remove them, you only need to find a bolt with the proper threading, screw it into the rocker arm shaft after taking the bearing block off, then pull the shaft out. You can then just take out the rocker arms.

[JD]

Seems as though when the bike is nice and hot that cylinder picks up a bit in the form of back firing. The idle will also race a bit for a few seconds and the whole engine noise changes and you can tell it's running better. Im not sure if the popping is just from excess fuel collected in the cross over or not. It will also smoke a bit from both pipes when hot. I did say that the bike pulled well when I first got it, but it had been sitting in my garage running for a bit. I have not opened the bike up yet as I wanted any issues to be known before I do. Thanks for the help--I'll keep posting and keep tinkering.

[Craig in France]

The carb bowl is full, as far as I can tell when I remove it. Ive played with the fuel taps, and fuel will pour freely from the needle valve when left open. Float level off? The plug is new and is not hot when the bike has been run.
JD

Hi JD,

OK, it seems like fuel is getting as far as the needle valve. So ...

When the cylinder hasn't fired, if you take the plug out, is wet or, at least, does it smell of gas?

If not, then no fuel (or at least, not enuf) is getting into the cylinder. Possible causes:

1. The needle valve and float assembly is hanging up, preventing fuel from getting into the float bowl. This can easily happen if the cover isn't correctly aligned. Of course, when you take the float bowl off, everything seems ok 'cos fuel starts flowing as soon as you start to loosen the bowl. But it's easy to check with the float bowl on - remove the main jet holder and fuel should flow freely!

2. (As you suggest) the float level is set wrong, cutting off the fuel flow too early. The real way to check is to reset it as described in the Dell'Orto manual (go http://www.filejumbo.com/Download/E9A7373D7DA0566F)

BUT, the float height adjustment would have to be WAY off to prevent enuf fuel entering the float bowl to feed the idle jet - this extends almost to the bottom of the float bowl.

Therefore, before you get into re-setting the float level, I would check this:

3. The passage that supplies air to the idle circuit is blocked, or partially blocked. This can happen very quickly - in less than 6 weeks once on my Laverda Corsa; and you may think you've cleared it, but it's still gummed up. Again, look at the Dell'Orto manual to make sure you know which one it is and then blow some carb cleaner down it until you see cleaner coming out of the air/fuel feeder hole downstream of the slide. You'll need to block up - just use your fingers - the other 2 drillings that feed into this circuit, i.e. the fuel supply itself and the drilling for the idle adjuster screw.

HTH, and let us know how you get on.

Ciao

Craig

[nottonight68]

if its popping/backfiring thru carby,its most probably a tuning issue.play around with mixture screw.most manuals or this site should explain the process

[wdietz186]

JD pay close attention to item 3 in Craigs reply. Take out the mixture screw and blow some carb cleaner thru the hole.With the float bowl off you should see cleaner blow out the idle jet and out the hole in the bell mouth of the carb.And most importantly with the carb pulled off the manifold you should see it come out the tiny hole behid the slide.these holes get blocked very easily when the bike has been sitting with fuel in the carbs.It forms a thick brown sludge that is pretty hard to remove. In extreme cases I've had to use a piece of very fine wire to break the crap loose.

[nottonight68]

any progress is the 432 now an 864

[JD]

No luck yet...It does run if I keep a slow but steady stream of carb cleaner running through the open end of the carb. Still going. Thanks.
JD

[nottonight68]

sounds like a needle/main jet problem to me
does the needle go up with the slide