loss of vacuum above 1000 rpm

[sggoat]

Hi all,
Got a strange prob w/my 77 FJ40. yesterday, everything was running fine-- this AM I go out, start it up - all looks good-idle at 700, vac at 20" smooth idle. Raise engine rpm to about 1100, and vac starts dropping little more throttle and vac falls to about 4-5" -- engine quits. restart again idle is great, vac good. run the rpm up again-same issue-this time pumped the throttle and got rpm up to 2000, vac oscillating around10-15", engine is surging in rpm about 300up/down. can increase throttle up to 2300 and hold the throttle still and rpm/vac surges get worse, but it's surging on it's own while I hold the throttle steady. lots of popping out the exhaust .
First thought it might be fuel--(this engine has a Weber on it- also have an electric fuel bypass pump to charge the carb bowl if the truck has been sitting idle for a week or so))-intake air filter good, fuel filter good. turning on the electric pump along with the main pump didn't change anything. ok, so maybe the points has changed gap a little--they were a little closed up, so filed them, and re-gapped to .018. Cleaned the rotor contacts and the cap contacts-- no change to the way it acts now I'm stumped--
Has anyone experienced these symptoms before? what fixes did you try? Where would you go next -compression test?
Thanks in advance for any suggestions

Gary

 

[CruiserInProgress]

While waiting for other suggestions, I'll offer an initial observation.

If the popping you mentioned in the exhaust is considered backfiring, I'm under the impression that backfiring in the exhaust header is the result of unburned fuel being ignited on the exhaust side of the combustion process, or a late spark (at higher RPM in your case).

Since it idles, you might check your timing with a light to verify you're in the range (7° to 14° BTC). Also, see if your timing advances as you increase RPM.

You might also have a look at your plugs to see if that reveals anything (inconsistent appearance between plugs, wet plugs, fouled, etc.).

 

[Pighead]

Did you have a vacuum line fall off somewhere?

 

[sggoat]

While waiting for other suggestions, I'll offer an initial observation.

If the popping you mentioned in the exhaust is considered backfiring, I'm under the impression that backfiring in the exhaust header is the result of unburned fuel being ignited on the exhaust side of the combustion process, or a late spark (at higher RPM in your case).

Since it idles, you might check your timing with a light to verify you're in the range (7° to 14° BTC). Also, see if your timing advances as you increase RPM.

You might also have a look at your plugs to see if that reveals anything (inconsistent appearance between plugs, wet plugs, fouled, etc.).

Not sure this sounds like a "backfire" but you may have a point with the spark advance. no major vac leak using carb cleaner spray around the manifolds or carb base - plugs coming out today(pics coming)

 

[JohnnyC]

Brake booster leaking? Try disconnecting the vac line to it (plug at manifold) and see if there lies the problem

 

[sggoat]

Didn't see any dangling hoses-all are connected-carb spray everywhere I could think of-only place that made even a little diff in idle rpm was around the brake booster vac connection( and that is very close to the weber carb inlet). just strange that the night before, the truck ran perfect-sat o'nite and first thing in the morn it has this issue. plugs out today, and maybe do compression check, check fuel flow to carb, and chk/repl filters

 

[sggoat]

Brake booster leaking? Try disconnecting the vac line to it (plug at manifold) and see if there lies the problem

that I will do today!

 

[JohnnyC]

to check fuel bowl level you can do it without taking off top plate by sticking a solid thin cardboard in the vent hole … see pic… don’t use wood as it doesn’t absorb to indicate level … my coffee stirrer is for visual example only … I just had a coffee and that’s what I had available lol

 

[sggoat]

to check fuel bowl level you can do it without taking off top plate by sticking a solid thin cardboard in the vent hole … see pic… don’t use wood as it doesn’t absorb to indicate level … my coffee stirrer is for visual example only … I just had a coffee and that’s what I had available lol





View attachment 4198518View attachment 4198519

this is great-didn't know it could be chek'd without dismantle

 

[Subterranean Nik]

Did you confirm timing yet?
Because it was running night before, would recommend looking for some of the obvious things that could have happened mentioned above… it will hopefully be a simple fix.

Unless a gasket suddenly failed or something sticking in the Weber, wouldn’t think that’s that issue IMO. I ran one for years and never had those symptoms, unless loose mounting bolts, old gasket, etc…

** as to timing, my 40 did some similar things with vacuum drop, “backfire”, etc…and it ended up being dizzy was in a tooth off (whoops). Bit don’t start pulling dizzy out - point being make sure timing where recommended above and or just let us know what it is showing.

Nik P

 

[sggoat]

not timing yet, trying to get at the easy stuff first- plus the truck was ok the night before(the dizzy is tight as to being able to turn it in the hole-vac advance plate operation is smooth). weber inlet fine screen filter was clean, next will chk for good fuel flow from the elect bypass pp. that is piped in parallel with the main pp(crankcase oil is clean with no fuel smell and at the same level it usu is)

 

[sggoat]

Fuel flow to carb is good using the elect pp---oops the video is too big--will take still shot so the flow is visible--plugs out next and comp check, then start rotating to get at timing marks

 

[JohnnyC]

Did you unplug the vac line to booster and plug … run it and see…

Unplugging booster to see if your leaking vac through the booster seal

 

[pb4ugo]

Opening the throttle plates will create a controlled vacuum leak, so vacuum will naturally go down as thr throttle opens. It seems like you may have an additional leak above idle. Spraying for a leak at idle probably isnt going net any results. You might turn the idle screw up or run the throttle up to the point when the issue happens then check for a leak. Id start by , checking the pcv valve, it might be sticking open. In high vacuum situations the valve should be open, as vacuum decreases the valve should be starting to close. If it doesn't close or seat completely in low vacuum situations it will create a vacuum leak. I'd also check the vacuum advance canister on the distributor by sucking on the line and see if it holds vacuum, also check the mechanical advance too.

 

[sggoat]

Brake booster leaking? Try disconnecting the vac line to it (plug at manifold) and see if there lies the problem

Found the booster line clamp broke and hose loose- prob why the carb spray caused the stumble-all fixed--try tomorrow to see if that helped

 

[Pighead]

Found the booster line clamp broke and hose loose-

So, just a loose vacuum line then?

 

[sggoat]

happy post--After the booster loose line thought I had it but same thing--pulled plugs--all good-compression all 120 except #6 low put back together and did more squirting this time_ when the booster area sprayed the engine quit--pulled the hose completely off--big dry rot crack in the back side of the hose tightening the clamp had made it worse new hose has fixed everything--all bad symptoms gone-- vac is ever higher at idle now--21"

Thank all of you folks for the tremendously helpful suggestions!!!!

Gary