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low vacuum on all 4 carbs

  • Thread starter Thread starter willsbike
  • Start date Start date
W

willsbike

Guest
I got this bike with the carbs removed. I replaced the rock hard carb holders already, bench synced the carbs. Backed out the idle mixture screws, also checked the valves and replaced the shims for proper clearance. Bike only runs on choke, and dies with throttle. I hooked up the carb sync tool
CARB_SYNC_KIT.jpg
All 4 gauges are in the red area that read low vacuum or improper valve timing. All in roughly the same area. I pulled the top of the air box off, and blocked it off with cardboard. I could get the bike to throttle up a little bit, and take the choke off, so I was thinking vacuum leak. But i sprayed carb cleaner around, and wasn't getting any raise in the idle. I haven't pulled the valve cover off yet to verify valve timing. Any ideas on where to start? It's the bike in my sig
 
"In the red area" is not much of a description. The actual number (or area on the gauge) is not as important as having all the readings the same.

Just wondering, what's the difference between the top of the airbox and a piece of cardboard? They both block the opening, but one is a bit more durable than the other.

Have you verified that your four gauges all read the same? With that type of gauge, you need to connect one gauge to cylinder #1, note the reading. Connect the next gauge to cylinder #1, note the reading, compare it to gauge #1. Continue connecting all the gauges to the same cylinder. If they all read the same, you can be assured that any differences are due to the engine, not the gauge. If there are differences, you will need to calibrate the gauges so they all read the same first, then proceed with the carb sync. A better gauge set does not need to be calibrated every time you use it.

How far out are the mixture screws? They won't affect carb sync, but they will affect how well the engine runs. Start with them three turns out. After the carbs are synched, turn them in until the engine speed drops, then back them out about 1/4 turn.

If the engine is running too slowly, turn the idle speed screw in. The manual says that carb sync should be done at 1500-2000 rpm, but I have never done it at that speed. I think the main reason for that suggested speed is to minimize the fluctuations in the vacuum gauge.
 
phydeauxmutt said:
"In the red area" is not much of a description. The actual number (or area on the gauge) is not as important as having all the readings the same.
Click to expand...

I'll get the actual number. Basically not enough vacuum to really register on the gauges. All gauges are reading pretty close to the same number though, it's just so low.

Just wondering, what's the difference between the top of the airbox and a piece of cardboard? They both block the opening, but one is a bit more durable than the other.
Click to expand...

The difference is the airbox lid has a hole in it, the cardboard has no hole in it. I blocked most of the air going into the airbox, trying to track down a leak.


Have you verified that your four gauges all read the same? With that type of gauge, you need to connect one gauge to cylinder #1, note the reading. Connect the next gauge to cylinder #1, note the reading, compare it to gauge #1. Continue connecting all the gauges to the same cylinder. If they all read the same, you can be assured that any differences are due to the engine, not the gauge. If there are differences, you will need to calibrate the gauges so they all read the same first, then proceed with the carb sync. A better gauge set does not need to be calibrated every time you use it.
Click to expand...

Good point about checking the gauges.

How far out are the mixture screws? They won't affect carb sync, but they will affect how well the engine runs. Start with them three turns out. After the carbs are synched, turn them in until the engine speed drops, then back them out about 1/4 turn.
Click to expand...

The screws are 3 turns out. I tried moving them in and out to see if it made a difference, but it didn't.

If the engine is running too slowly, turn the idle speed screw in. The manual says that carb sync should be done at 1500-2000 rpm, but I have never done it at that speed. I think the main reason for that suggested speed is to minimize the fluctuations in the vacuum gauge.
Click to expand...

1500 is about where I had it.

Before I even tried to sync it, the bike would only run at full choke, and dies with even a little bit of throttle.
 
willsbike said:
Before I even tried to sync it, the bike would only run at full choke, and dies with even a little bit of throttle.
Click to expand...

You're trying to synch the carbs when you haven't even gotten them working yet?
Take them out, take them apart, clean them thoroughly.
If it won't run without choke your pilot circuits are clogged, and probably a lot more.
It shouldn't ever need full choke, except maybe starting up cold in sub-zero temperatures.
Do it right, short cut are not short.
 
tkent02 said:
You're trying to synch the carbs when you haven't even gotten them working yet?
Take them out, take them apart, clean them thoroughly.
If it won't run without choke your pilot circuits are clogged, and probably a lot more.
It shouldn't ever need full choke, except maybe starting up cold in sub-zero temperatures.
Do it right, short cut are not short.
Click to expand...

The carbs were rebuilt before I got the bike, so I didn't start with that in the first place. I threw the carb syncer on there before I realized I still had a problem. Since all carbs were measuring at 3" of vacuum, I didn't think it was a carb problem. After today, though, I found a vacuum leak, so I'm thinking it was that, plus clogged carbs, so that will be next.
 
Alright, so #1 cylinder is at TDC, and I count 20 pins between the 2 and 3 arrow. Is this correct? the 1 arrow looks to be lined up correct.

Also, I found that the 2 drain hoses from the carbs are sucking in air. I was getting 3" of vacuum on all 4 cylinders, with them blocked I was getting 7 or 8" on 2 cylinders, and around 3 or 4 on 2 of them.

I backed out the idle screw all the way, and the bike still runs. Even with those hoses blocked. Vacuum leak still?
 
I also noticed there was no mention of cleaning the carbs. You really want to believe someone who says the carbs were cleaned? Unless you do it yourself, you will never know what you have for sure.
Do the berryman's carb dip or else ultrasonic cleaning then start over from a good working baseline.
As tkent02 says, short cuts are not really short, just more work in the end.
 
I got the bike from a friend, I had already bought the bike when he said he had done it. But you're right. I don't know the extent of cleaning. I'll take them apart and throw them in the sonic cleaner. Any recommendations on solution for sonic cleaning?
 
willsbike said:
I got the bike from a friend, I had already bought the bike when he said he had done it. But you're right. I don't know the extent of cleaning. I'll take them apart and throw them in the sonic cleaner. Any recommendations on solution for sonic cleaning?
Click to expand...

Doesn't really matter, any old detergent works, you won't be leaving in long enough to discolor anything.

I used Awesome detergent from the dollar store, and it was awesome.
 
Check all four compression numbers on a warm motor with the throttle open fully.
 
what was more than likely happening with the drain hoses drawing air is from running off of the choke circuit. the whole carb draws fuel via engine vacuum, and when the rpms run high, you start hitting the main/needle jet, and that's probably where the vacuum draw is coming from. just give those carbs the full strip and dip and that should minimize quite a bit. I'd pop on down and lend a hand if you weren't so far down south from me.
 
Following
I have the exact same problem, except I had my carbs cleaned. Starts, but dies if i touch the throttle or take of the shoke.
 
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