Miss in 409 # 5 cyl.

dualquadpete · Canadian Poncho Superstar!

Miss in 409 # 5 cyl.

4SPEED427 · A Poncho Legend!

Crazy as it sounds, maybe a vacuum leak?

Years ago I had a 396 with a miss at idle. Narrowed it down to #8. In the end, I forget exactly what was leaking but it was a vacuum leak that was causing it.

Greaser · Canadian Poncho Superstar!

Had a friend who had the same issue a few years back. Car sat for years without much use.

A can of Seafoam in the oil cleared up all the nasties in the block.

After the Seafoam followed by an an oil change car ran smooth at idle.......still does.

Follow the instructions on the can, cheap fix if it works.

-G

Prefectca · Poncho Master!

Check the firing order. I have seen many Chevies back in the day with plug wires #5 and 7 crossed and run well but have a miss at idle. Also check the distributor cap for carbon tracking between 5 and 7. Also try another spark plug. I have seen plugs fire when out of the engine, but still be dead when in the cylinder. The big brand name that starts with C come to mind. I have seen lots of this brand fail on cars that were in long term storage. Plugs that look new, but wont fire when in the engine.
Paul

Magician40 · Poncho Master!

Vacuum leak is a good bet. Also 185lbs is high 150 about right, did you do a wet test as well to see if you had a ring or valve leaking. Bent pushrod, or my constant problem when I had my 409 was valve springs and keepers going bad. Worn cam face too. Was the miss there before the distributor change?
Brian

dualquadpete · Canadian Poncho Superstar!

Magician40 wrote:
Vacuum leak is a good bet. Also 185lbs is high 150 about right, did you do a wet test as well to see if you had a ring or valve leaking. Bent pushrod, or my constant problem when I had my 409 was valve springs and keepers going bad. Worn cam face too. Was the miss there before the distributor change?
Brian

YEP, been there since I bought it, changed S/plugs [ AC Delco] wires points & condenser,cap rotor etc. then did the HEI conversion. have swapped plug from another "firing" cyl. same thing. Put a scope down intake this morning,tight fit but I think that passage is "blocked" just ordered a intake gasket Pkg. [ looked like it may be packed with "furry" stuff, Mouse nest"?] will get it off ASAP but have company coming from West Virginia tomorrow and busy getting ready for them & 70 Cutlass !!! Also going to contact previous owner to see if he had carb. off at some time during storage???

dualquadpete · Canadian Poncho Superstar!

I had the R/C off and everything looked good, Ran it & rockers were working, no bent P/rods no broken springs. also ran propane torch over intake, and carb base etc. No Change ,idle still rough.

JC2+2 · Poncho Master!

Remove pcv valve and check if there is vacuum in the crankcase. Vacuum leak can also be internal. Seen it before but mostly on Fords.

JC2+2 · Poncho Master!

Also check the transmission Mod valve for leak. Doesnt that go to number 5 port.

dualquadpete · Canadian Poncho Superstar!

No Modulator valve !!! 3 pedal car !!!! Just took a minute & re checked firing order A-OK Earlier I used my Laser heat gun & that cyl [#5] definitely not firing, much colder than others.

JC2+2 · Poncho Master!

Did you swap plugs with a good hole just because? And did you check for booster leak? You never mentioned if you checked for the internal leak as well.

hawkeye5766 · Canadian Poncho Superstar!

What does your vacuum gauge read??? Is the needle bouncing? Timing light on #5 with consistent blinking?

68vert · Guru

Sorry I don't have much to offer but very interesting and informative following everyone's troubleshooting ideas. Hope one of them gets it resolved for you!

DonSSDD · Canadian Poncho Superstar!

Try a dose of sea foam in the gas? Would clean out carb and intake, valves?

What did #5 plug look like when you pull it?

No miss except at idle?

What does vacuum read and is it steady?

Sounds like youve eliminated everything electrical so it has to be fuel. Got another carb to swap on there? Bore size same as Edelbrock or Carter. Has to be carb or plugged intake passage?

Magician40 · Poncho Master!

Hope the intake gasket change solves your problem cause what you saw on your scope probe seems like the answer. Guess a 7cyl. 409 would be a fairly equal run against a 327 now. I saw your post on the 70 Cutlass, are you teasing us Pete.
Brian

dualquadpete · Canadian Poncho Superstar!

Yes I swapped plugs, no change!! No booster,manual brakes, Vac. gauge steady & good Vac. Also gave it a "dose" of "Rislone" down the carb, and "snuffed engine out, & let it sit for 3 days, then drove the "snot" out of it. Some "crap" out the pipes!! Car has a slight miss at cruise "55mph" with light throttle. & 5 showed a intermittent "miss fire" with a tester on it. Under full throttle It pulls like a freight train!!! Intake coming off maybe today if time permits.

4SPEED427 · A Poncho Legend!

dualquadpete wrote:
Under full throttle It pulls like a freight train!!!

And that's the weird part, the part that makes it sound like a vacuum leak.

I think a lot of us are really curious what the solution will be!

hawkeye5766 · Canadian Poncho Superstar!

dualquadpete wrote:
Car has a slight miss at cruise "55mph" with light throttle. & 5 showed a intermittent "miss fire" with a tester on it. Under full throttle It pulls like a freight train!!! Intake coming off maybe today if time permits.

Why look for a vacuum leak before you figure out the miss fire? Timing light blinking should be consistent. Portronics fires off of the dizzy cam lobe. Had a problem way back with that setup in my 57. Would miss and sometimes pop through carb. If your distributor has a worn lobe the portronics may not pick up the signal as it's too far away. Also, if your dizzy bushings are worn or vac advance plate this could cause a miss fire. Another thing to check. hmm

-- Edited by hawkeye5766 on Thursday 8th of August 2019 11:50:57 PM

dualquadpete · Canadian Poncho Superstar!

Got intake off, no blockage, but the imprint of gasket on # 5 on lower section is "not" there !!!! Had to use "johnson bar" on bolts to crack them loose!!! Going to check intake with straight edge!! Air gap on " pick up is with in specs. on full 360 deg. turn & up & down movement too;

4SPEED427 · A Poncho Legend!

I can't remember, is it a stock type iron intake?

dualquadpete · Canadian Poncho Superstar!

Yuup, cast single 4 BBl.

JC2+2 · Poncho Master!

So pretty good chance it was an internal vacuum leak then. Did it actually pass the test?

dualquadpete · Canadian Poncho Superstar!

Had carb. off so couldn't do that check, Also going to scope down the intake port on head likely next week as I gave it back. nothing showed on propane torch test either on intake or carb base. Still not convinced gasket is problem & will roll engine over & check cam lobes, "plopped" Dist. back in. Pull R/cover & check again????

Prefectca · Poncho Master!

As far as the intake gasket not sealing, I have seen this happen if the heads have been planed or the block has been decked. The angle of the heads change slightly causing the gasket not to seal. If the block has been decked and the heads planed the it becomes a bigger problem. If I recall for popular engines there were thicker gaskets available.

Paul

dualquadpete · Canadian Poncho Superstar!

I've checked down the heads intake & exhaust ports, little bit of "gunge" on valves but nothing excessive!!! Rolled engine over & cam "lift" equal to others??? Into "another " job now !! Friends arrived from W. V. with a bad brake pull on 70 Cutlass, adj. frt brakes, right side way out of whack, stopped straight in town, went down #57 & hit the brakes at 80km. "BANG" Rt. frt. brake drum "EXPLODED" !!! called girlfriend to come & get me, went back with trailer & got car home. On removal of wheel the whole frt of from with stud holes came off, & huge chunks of drum missing, just have a drum with no centre !! Have parts on order for this afternoon.!!!!