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Home » Public Forums » GMCnet » A couple 'issues' I'd like some advice on. (A 'cough' and low vacuum)
A couple 'issues' I'd like some advice on. [message #270901] Sat, 31 January 2015 19:55 Go to previous message
kerry pinkerton is currently offline  kerry pinkerton   United States
Messages: 2565
Registered: July 2012
Location: Harvest, Al
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Senior Member
I've got a couple things going on and I'd appreciate some wisdom from the brain trust. I have some ideas but I'm scratching my head.

Some history and then some facts.

77 403 26' pull a Saturn toad and have a Powerdrive and a 3.21 which ends up as a 3.66 final drive ratio. 65mph is about 3000 RPM.

Motor was rebuilt about 5000 miles back, As of yesterday, compression is 150-165. All but 1 cylinder (#4) is between 165-160. Patterson carb and distributor with his plugs and wires replace when the engine done. All electric fuel pumps. Two pumps outside the tanks, filters before each pump, another filter before the engine, and the carb filter. Switching to the other tank changes which fuel pump is on.

A couple months back, I replaced the timing gears with a 4 degree advanced set. At the same time, I changed my vacuum gauge from an elcheapo gauge that had a range from 10 to 2 o'clock, to a S&W guage that sweeps from 7 to 5 o'clock. Readings on the vacuum match a large old mechanics vacuum gauge that was my Dad's back in the late 30's and has always been very accurate.

When I first started the engine after replacing the timing chains, it had been sitting for a couple months while I replaced the tranny. The motor idled like crap. Rough, popping through the carb. It would smooth out as it warmed up but still ran a bit rough. Yes, the modulator vacuum hose is on and functioning correctly. I talked to Dick Patterson and went through the timing, dead on 12 degrees with the vacuum adv off and plugged. Verified the mechanical advance is working, also verified the vacuum advance is working. Dinked with it a bit over a couple days and things got better as far as the idle was concerned but never really changed anything. I moved the timing a degree or two but was at 12 when I started and ended up at 11. Backed out the idle screws about 1/4 turn. About this time I drove the coach and noticed the vacuum seemed a bit low. 55 mph steady and level was about 10 inches of mercury. Hot idle was about 16.

Talked to Dick again wondering if the advancing the cam timing 4 degrees would effect the vacuum. He said no and thought it was a vacuum leak. I dinked with the timing and got it up to about 12 at 55 mph steady and level road. I've checked everything I can for vacuum leaks. New PVC valve, removed the booster vac line and plugged it. No change on vacuum gauge. Carb bolts are tight, modulator is hooked up. Everything is capped. The fact that I've got 18 at hot idle makes me think it's not a vacuum leak because I'd THINK that a big vacuum leak would show up more at idle than at higher RPM?????

Problem #1 is the vacuum level. The coach runs fine. Pulls stronger than before. BUT at 65mph on a level road just pulling against the wind and maintaining speed, my vacuum is 7-8. I watched it all the way to Tampa and back. Runs fine, just lower vacuum than I thought it was before. At 55, level and steady, I read 12-13. At 65, level and steady but obviously a heavier wind load (don't forget I'm towing too), I'm at 7-8. Btw the toad brakes and coach Ebrake are clear and rolling free.

So the question is, what kind of vacuum do you get a 65 level and steady? Dick said since the coach was running fine, I should put a piece of electrical tape on the vacuum gauge and just drive it. He might be right but I THINK it was about 5 inches higher before I advanced the cam timing. I don't THINK advancing the cam timing should effect the vacuum much if any. If I set the timing for max vacuum, I can get hot idle vacuum up near 18-19 but the timing mark is almost out of sight...probably 25-30 degrees of advance and that's with vacuum off and 700 RPM.

Problem #2 is a "cough". Not a backfire, just a 'chu'. Happened a couple times on the way to Tampa and 6-10 more on the way home. Its not much, just enough that the wife sometimes asks, "What was that?"

Remember the rough running that seemed to go away right after I got it running again.

One more data point. On the way home, we stopped for lunch and I started the Onan. Ran like crap. Wouldn't come to speed and stabilize enough for the automatic transfer switch to turn on the 120. Turned it off and came on home. Now it starts and runs BETTER but still rough, it will at least trip the transfer switch on. My buddy Beau thought it was crap in the carb.

I'm wondering if it is water in the tanks? This trip had 1600 miles of buying today's ethanol laced crap. Other than one tank starting out, and a stop at a big Shell in Florida (Ken and Stick also filled up there and have reported no problems) all the gas was purchased at big truck stops, Flying J, Petro, etc. I'd have thought that any water would have been flushed out,

I've thought about plugged fuel filters. Wide open throttle is no problem and I'd think it would be if there was a partially blocked filter.

It's not repeatable. 10 times in 1600 miles. It SEEMS to happen when I'm transitioning from a level pull or downhill to more throttle at about 2600 rpm. Just a little 'chuu'. Not a bang, not a pop, not loud, literally sounds like a muffled cough.

So today, I checked the timing again with TWO timing lights. 12 degrees.

Ken Henderson loaned me a wide band O2 sensor and I welded a bung up close to the engine and hooked it up.
At cold idle (700 rpm) with the choke about closed it read 10.4
warm idle with half choke was 12.1
hot idle (hot enough that the oil cooler fan was on so the oil temp was > 180), the ratio was 12.4 climbing to 13.2 as it ran a bit more.

IN PARK (it was getting late and I did not drive it today)
700 RPM 13.2
1300 RPM 12.0
2000 RPM 14.6
2500 RPM 15.7
3000 RPM 15.9

That's all the data I have.

In summary, I have two issues. They might or might not be related:

1- Low vacuum? (or perhaps it's just fine???)
2- The cough? Water?? Add a couple bottles of denatured alcohol??

Your thoughts, advice, etc are solicited. Please try not to hijack this thread. If you want to talk about something else, please start another thread.

Kerry Pinkerton - North Alabama Had 5 over the years. Currently have a '06 Fleetwood Discovery 39L

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