petcock tested fine but no vacuum from Carb?

[Jackw]

So I tested the petcock as described in the "tips and Tricks" section. It works as it should but the carbs aren't creating vacuum when the bike turns over. I don't get any fuel out of the tank when it's hooked up as it should be. Just had the carbs rebuilt? My question is. if carbs are plugged will they create vacuum? or is this strictly an air issue? like an air leak somewhere? or am I way off?

Thanks for any advice
Jack

[onchiman]

Does the bike run?

[Steve]

There is a fitting on carb #2 that should have the vacuum hose on it during normal operation. While the engine is running (can you get it running in the PRIME position?), disconnect the vacuum hose from the petcock, leaving it attached to the carb. You should feel vacuum pulses at the end of the hose. If you don't feel any vacuum, remove the hose from the carb, too. Try blowing through the hose, it might be plugged. If the hose is OK, that only leaves the carb fitting. If there is something plugging the opening, it would be large enough that I would not want it going through the engine, so ... pull the carbs, poke a wire through the vacuum fitting to clean it out.

Since you say that the carbs have been recently 'rebuilt', I would put my money on a collapsed vacuum hose. 8-[


.

[Jackw]

Thanks guys

The bike will not start, It was running fine until the other day then it just didn't start. Checked for a vacuum pulse last night with the hose on. didn't feel anything. Vacuum hose is new and very rigid. I know that the hose isn't collapsing. pulled the carbs last night so I'll check for blockages tonight. Tank was acid washed during carb rebuild maybe it wasn't rinsed well and some dirt got into the carbs? I pulled one bowl last night and there was some very fine dirt in it.

[Nessism]

So I assume that gas will flow if you suck on the vacuum line going to the petcock? If so and you are still having problems, maybe you have the vacuum like hooked up to the wrong port? The proper take off is located on the #2 carb but your bike may be different. http://www.thegsresources.com/images...%20Petcock.jpg

[Jackw]

yup, gas flows when you suck on the hose and it flows on prime. Bike won't start on prime, but that's another issue in itself. Just need to get fuel flowing then worry about the rest. Will an air leak around the boot that connects the air box to the carbs cause vacuum issues? or do the carbs automatically create vacuum regardles of other problems?

[onchiman]

I doubt you'd feel the vacuum while just using the starter. I've tried it before and couldn't detect it. When the engine is running it should be fairly easy to feel. Since fuel flows on prime, you know your getting juice to the carbs. How do the plugs look, are you getting a healthy spark?

[Jackw]

No spark isn't healthy at all, that's my next step once I get fuel flowing. With everything hooked up properly fuel doesn't flow while cranking the bike and since the petcock operates properly and the vacuum line is new and not colapsing I assume there is a carb or air flow problem

[onchiman]

Don't get hung up on the petcock at this point. If it's getting fuel on prime, we need to figure out why it won't start. If the spark is weak as you say, start there. Battery fully charged? Spark plugs fouled?, timing correct?.

[Jackw]

ok man I'll work on fire first. I think it may be the coils. I took the end off of the plug wire and held the bare wire aobut 1/8" away from the block. Fire still seemed weak on one coil and other was firing intermitenly. The coil with the weak fire worked the same on both coil connection. is there anything behind the coils that'll cause weak or no firing? can I test the coils by testing their resistance or voltage output?

[onchiman]

The battery is behind the coils in the process. Make sure the battery is fully charged. Also, clean all connections and grounds, check the breaker points (if it has them) and timing. Once you get a good spark at the right time, and verify good compression in all your cylinders, you can move on to the carbs You mentioned crud in the carb bowls too, that's not good either. Looks like you got some work ahead.

Internal combustion engines need three things to run;
1. Proper fuel/air mixture
2. Adequate compression of the fuel/air mixture
3. A spark delivered at the right time

[hikermikem]

[quote=Jackw;675841]ok man I'll work on fire first. I think it may be the coils. I took the end off of the plug wire and held the bare wire aobut 1/8" away from the block. Fire still seemed weak on one coil and other was firing intermitenly. The coil with the weak fire worked the same on both coil connection. is there anything behind the coils that'll cause weak or no firing? can I test the coils by testing their resistance or voltage output?[/quote]

Your coils should test as follows:

Primary resistance (between "+" tap & "-" tap) = 3-5 ohms.
Secondary resistance (between plug caps on same coil) = 30,000 - 60,000 ohms.

Radio Shack sells a very nice digital multimeter for under $20. The battery for it costs ~ $7. Good to have one of these in your tool arsenal as you can check many electrical items on your bike.

With that said, sometimes the coils bench test OK, but still are defective when under load. The wires and the plug cap connections can be broken and/or corroded.

I had an '85 550L and the coils bench tested OK, but the spark under load was messed up. The bike ran well at idle & low RPM, but at higher RPM it stumbled & broke down. I put new aftermarket coils on and the difference was amazing - fixed a whole lot of issues.

http://www.z1enterprises.com/detail.aspx?ID=210

Good luck getting her going.

[jpaul]

You won't lose vacuum at the carbs with a leak in the boots between the airbox and carbs. If you don't get sufficient vacuum to open the petcock, even with just the starter motor, you most likely have a leak between the carbs and the engine. If you have the original boots between the carbs and the head, I would replace them. New ones are still available, and not that expensive, expecially when you consider it's a once every 25 year replacement.

Just looked at the parts fiche and saw that your bike has o-rings between the carbs and the head. Those are likely the problem if you're not getting vacuum at the carbs. I think someone on here was putting together o-ring kits to do that job. Do a search to see if they are the same ones used on your bike. If not, the o-rings are about $6 each.

[chef1366]

Check compression..............

[Jackw]

Thanks for all the info guys. Compression tested between 130-140 PSI on all cylinders. I'll check the intake pipes and O-rings between the carbs and engine over the weekend and get those coils tested as well. Hopefully next time I post here it'll be to thank everyone and let you all know how sweet she is running(fingers crossed). Have a good weekend All

Jack