Maybe valves...what can accomplish without removing the head

Background:

861 was running a little rough and very wet leaky carb. Belches/backfires through the carb when giving it gas. Rebuilt carb and replaced graphite wires and new points and IC14 coil. Pulled and cleaned distributor. Timing is right, confirmed with light.

Idles OK and runs high rpm OK. But, giving it gas rapidly at any speed still causes a belch or two or three through the carb. GB in MT and I have conferred and looked at pictures, etc. We believe the carb is setup OK and rebuilt properly. So next we discussed checking the vacuum.

Vacuum is 17 inches, but I do have a needle bounce. And if I give it gas quickly the vacuum drops to 0-5 inches before recovering. AH HA! So I'm pretty sure that there is a valve issue causing the belch.

Oh...have 20 lbs oil pressure at idle, 40 lbs if 1000 rpm or higher. Not sure about oil pressure when working it. Haven't gotten that far. So had no plans to go into engine, if it can be avoided.

Tin and gas tank are coming off for cleaning, wiring and paint anyway. [b:4fac2a7575]So how much, if anything can I do with valves without pulling the head? I'm guessing adjusting, but that may be it?
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"I'm guessing adjusting, but that may be it?"

Pretty close, but then, that may be all you need.
Always the optimist!

2 things.

check for a loose rotor.. then do a comp test..

the needle bounce at idle... could be valves. or could be worn dist shaft, loose rotor.. put a dwell meter on it.. if dwell is steady then look at valves...

when you dump the throttle... mixture goes lean and if it does not have accelerator pump, it will burp, belch, fart, and cough.... running the idle mixture a bit rich will help that.

Try a running compression check.. google it...

I'll double check the rotor. Distributor play seemed OK, but I didn't actually measure it. Dwell measured low, if I remember...in the 30's. But I didn't put a whole lot of faith in that because the meter does have electronics in it. I can look for consistency, though.

Initial compression testing, before any work was done was:

165/175 145/160 160/165 155/175

It had sat for a year not running with PO. I had noticed sometimes reading 20 lbs oil pressure and sometimes higher. Have treated cylinders with ATF and since have recently added ATF and Seafoam to oil and run it at idle. Oil pressure is more constant now. Will change that out and redo compression test with fresh oil and filter.

Running rich does help a little. Fewer belches, but not yet right.

Had an 861 and a 961 both would falter,slight hesitation,cough, on rapid acceleration. Never did backfire. Raised the float level on both about 1/8 inch. Solved them both. Just a thought. HTH

If you have the right equipment, you can replace springs, keepers, umbrella seals (if equipped which is doubtful). I don"t think that would fix any of your problems though.

It's not valves.

Sounds like carburetion. Richen it a bit.

Normal for most dwell meters to fluctuate a bit on a 4 cylinder engine at low RPM.

Could be irratic dwell/timing due to worn distributor bushings, cam, advance mechanism, etc., but I suspect carburetion.

Dean

It's lean.

Probably didn't say that right. The float level is normally is about a 1/4 inch. I set them at about 1/8 inch.

"And if I give it gas quickly the vacuum drops to 0-5 inches before recovering."

As it does with ANY other intake throttled engine when you pop the throttle open!

NO "smoking" gun there, Beanie!

Timing is right, confirmed with light.
Yea but at what RPM did you check the timing. It makes a difference.

if I give it gas quickly the vacuum drops to 0-5 inches before recovering
That's normal so what is the problem

Dwell measured low, But I didn't put a whole lot of faith in that because the meter does have electronics in it.
Well there is a smoking gun but you blew it off as a bad gauge. If your dwell is off your timing is off.

165/175 145/160 160/165 155/175
With readings like that I would be real hesitant to go very deep in that motor chasseing a minor hesitation on acceleration even though #2 may be a LITTLE weak.

Running rich does help a little. Fewer belches, but not yet right.
You may never get it perfect. This is not a race car; the carb does not even have an acceleration pump so you may never get all the hesitation out of it when "giving it gas rapidly" as you put it. Running it a little rich at idle does help some to overcome this design flaw.

Have you checked the mechanical advance even though it is pretty hard to check other than sticking. Weak springs on the advance will throw off the timing while accelerating.
If you really want to get spunky and check every possibility do a dynamic compression test like Hobo suggest. This will tell us if you have a blockage in the intake and or exhaust that could result in bad breathing problems for the engine.

yes, you can adjust the valves, and if you haven't since you bought the tractor, they need it.

Then, easy first.
forget your bog, until you get the ignition perfect.
When timing those engines, get the advance specs from the
manual and mark the max advance on the flywheel and then use your light at the proper rpm to nail it exactly.
IMHO, the initial at idle is nowhere near as important as having the max advance right at the right rpm.
When the ignition is perfect...

You have described how I set the carb on my tractors.
driving on a trail in 3rd gear at near idle, hammer the throttle to wide open. If It stumbles, I'm too lean.
If small adjustment won't stop it, back into the carb...I missed something.....

Maybe the dwell was the smoking gun. I need to find the proper readings in the book again.

But the reason I said the timing was right was that I found TDC on #1 and "wiggled" the engine to 4 degrees BTDC and the rotor points to #1 on the distributor. Cranks good and shows 4 degrees BTDC at 450 rpm.

The advance did concern me too. That's why I removed the distributor and cleaned all the crap out of it. Weights are free, but I wonder about the springs being weak. I know it is advancing, but haven't yet been able to spot the specific degree marks on the flywheel for checking at the higher rpms. I'll work on that more.

Didn't know that amount of vacuum drop was normal with rapid gas increase. I learn every day! :lol:

Dwell is an indicator of point gap, if dwell is off then gap is off. Get that right, then set the timing to the book. A change in gap changes timing, so no good setting that until the gap is right.

17 inches sounds low on idle vacuum, the pulse could be a miss or valve issue.

1. get point gap and dwell right.
2. get timing set to book specs
3. adjust valves
4. adjust mixture

Another thing dwell can show you is a worn distributor bushing. The dwell should be constant if the bushing is good, but vary if not.

I do not put much stock in dwell, get your gaps set right.

If dwell reading is somewhat steady... you dont have a bad dist bushing.. or loose rotor...

a worn out dist will give you a wondering dwell reading,, all over the map.. steady dwell means good dist..

dwell reading itself is the time the points are closed and means that the gap is off a bit... you can overcome that by timing, but the correct dwell or gap to start with is important for proper coil charging... so set dwell or gap first, then time second... Set idle jet a bit to the rich side, if needed set idle a bit higher.. both will help it "get off the line" guicker on a carb.. especially when it goes from normal to extreme lean till the vaccum can catch up... seting main jet a bit rich helps also..

if the vacum has minor bounce,, then could be compression differences... worn lobe on cam, valve adjustment all over place.. your compression check does not show burnt valve or valve staying open...

as others said.. go down the numbers and put everything right...

idle it up a bit will help also..

Does it have the little spring clip under the rotor?

Yep, has the rotor spring.

Well, I remembered that I had an EI and that should take care of some distributor wear. Also handled the dwell question. Popped it on and rechecked the timing. No improvement in popping and belching.

New clue though. I was removing the rubber tubing that connects to the fitting added to the intake manifold for checking vacuum so I could give a shot of JB Tuneup, which is supposed to help if sticky valves. Surprised that the engine sped up and smoothed out some before I even used the JB Tuneup! Talked to GB in MT and I'll be going back in the carb to see if I can find a blocked passage. I am clearly not getting enough carb air flow.