79 inline blew again

[jimreinarts]

Hi I had my 79 inline 115 rebuilt ,because of the #3 piston going bad the rest of the engine was ok ,after rebuild i had problems from the get go with aluminum deposits showing up in the plug hole, backed off timing and it blew again same as the first time, the other 5 cylinders are great shape and rich , i jetted up after rebuil from 74s to 80s , the rebuild had all new pistons and bearings.

[ChrisCarsonMarine]

Too bad...the timing advance changed from 23 to 21 degrees years ago,realize you must set the timing pointer with a dial indicator to start with.I think the jet change should be about right for a 115,a little lean for the 140-150.Did you replace the pistons with high or low dome pistons?Number 3 has the most timing,and will be the first to detonate,all other things being equal.This motor was designed when gas was gas,and is octane-sensitive,even the low dome motors need premium fuel.Your motor suffered low level detonation,possible causes include low octane,high timing,over propped[same result as over timed these motors need to turn up,5500-6000 for the 115,6000-6500 for the 140/150]lean fuel mix[jetting]wrong spark plugs,or a combination of the above.Also,i think the 79 still uses a distributor,if so,check the cap for brush dust,moisture,or carbon traces,these can cause the spark to go to the wrong cylinder,same results,chris

[Bruster]

There was also a new timing window (at some point and time) for the distribuitor that retarded the timing of #3 to fix this problem.


Too bad...the timing advance changed from 23 to 21 degrees years ago,realize you must set the timing pointer with a dial indicator to start with.I think the jet change should be about right for a 115,a little lean for the 140-150.Did you replace the pistons with high or low dome pistons?Number 3 has the most timing,and will be the first to detonate,all other things being equal.This motor was designed when gas was gas,and is octane-sensitive,even the low dome motors need premium fuel.Your motor suffered low level detonation,possible causes include low octane,high timing,over propped[same result as over timed these motors need to turn up,5500-6000 for the 115,6000-6500 for the 140/150]lean fuel mix[jetting]wrong spark plugs,or a combination of the above.Also,i think the 79 still uses a distributor,if so,check the cap for brush dust,moisture,or carbon traces,these can cause the spark to go to the wrong cylinder,same results,chris

[M. HARDEE]

WHAT THEY SAID and make sure it has a good strong water pump . any extra heat along with the obove would compound the problem.

[wireless]

Number 3 has the most timing,and will be the first to detonate,all other things being equal.

I remember something about #3 having the best tuning on the exhaust side, so it works the hardest of all 6, and for sure is the one that detonates first. If memory serves me, #1 is next in line (no pun intended) for detonation.

Absolutely set the plastic pointer with a dial indicator off #1.

Also, snap-timing with a dial indicator is far more accurate than trying to do with a light (and you don't need the pointer or flywheel decal or marks). We saw a number of flywheels on in-lines where the timing decal was misalligned by as much as 3 degrees (some too far advanced, some too far retarded).

Also make sure that the timing belt / distributor pulley is properly alligned - but I think if you are off by one tooth on the belt, you would not get even close to setting the timing without huge, crazy adjustments - so that is probably not an issue.

If the distributor was ever rebuilt - like to replace the trigger, make sure that the trigger (window) wheel was put back with the proper side facing the rotor. If you look closely at that wheel, you will notice that the windows are different sizes and have different timing trigger points per cylinder. I believe you can cause havoc if this wheel is flipped over by mistake.

[jimreinarts]

Thanks for the replies, i will give you some more info, my 79 does not have a distributor it has the switch boxes, also was rebuilt with hi dome pistons ,i had the timing at 16 degrees instead of the 21. i run it from 6000-6400 chopper prop 28=14 1/2. also using the short mid, with the lower unit holes pluged other than the enlarged bottom two, the gauge showed 7psi at the time it started siezing. chris how can i build this inline to last this time around. being that i will need a piston i could buy a low dome and even shave all six down even more, i was using 91 octane no ethanol and 40to1 syn blend pennz. i already have 8 grand in this viper and if i go through the trouble again to rebuild i guess i would like to know the best way for the fuel we have now.again thanks guy's for the help.

[Raceman]

Thanks for the replies, i will give you some more info, my 79 does not have a distributor it has the switch boxes, also was rebuilt with hi dome pistons ,i had the timing at 16 degrees instead of the 21. i run it from 6000-6400 chopper prop 28=14 1/2. also using the short mid, with the lower unit holes pluged other than the enlarged bottom two, the gauge showed 7psi at the time it started siezing. chris how can i build this inline to last this time around. being that i will need a piston i could buy a low dome and even shave all six down even more, i was using 91 octane no ethanol and 40to1 syn blend pennz. i already have 8 grand in this viper and if i go through the trouble again to rebuild i guess i would like to know the best way for the fuel we have now.again thanks guy's for the help.

Jim, I've always run inlines EXACTLY like you're describing and have never blown a powerhead, and I've always run high dome pistons. There's only .100 difference in the dome height, with no other differences. Back when the engines were current, and the gas wasn't the watered down crap we get today, I'd run premium, which was upper 90's octane and old Merc oil at 25:1. Nowdays I run my stuff at 32:1, although I think 40:1 is more than adequate with today's full synthetics. Just for safety now I put in about 25% race gas BECAUSE: I'm too lazy to take a chance with fryin' one, I don't trust the crooks at the gas stations to sell pure premium just 'cause that's what the tanks say, AND I've got it layin' around for the old cars anyway and don't run an inline enough for the cost to make a difference. Dangerous Dan, who's been runnin' a 1500XS on a 15 Ally around here and in that now defunct Florida drag race series told me the other day that he's run regular in his some and gotten by with it. (unfortunately he popped a steering cable pulley mount and tripped the boat, breaking his leg, so gas won't be an issue until some BIG glass work is done)

Anyhow, just grasping at straws 'cause it sounds like you're doin' all the right stuff. I think I'd have to index the flywheel and see what's goin' on with the timing (makin' sure your timing mark is in the right place) and if that doesn't raise any red flags be lookin' for some fuel/carb issue in the middle carb. How'z it jetted? Still thinkin' on this one.:confused:

[sschefer]

Any chance it's a bad reed block? A leaker could lean it out and it would repeat itself no matter how many times you replace the piston.

[jimreinarts]

Its jetted with 80 mains i know that much, when i do rebuild again i will try and find someone that will go through the carbs that was also a thought of mine, as far as the reeds they were good at the rebuid time from what the guy told me that assembled the engine but that could also be an issue , i was considering trying fiber reeds , a friend of mine from minneapolis is rebuilding one of his 1500xs motors with them i will see what he has to say. the timing, that was done by a classmate of mine that is a mercury mechanic it was checked with throttle w/o at cranking with the starter speed we started at 21 , the motor ran great at break in time of course then when i would start to do a max run after break in ,it was almost like the motor was dropping plugs above 70mph and 6000 rpm , leaving aluminum deposits on more than one plug, i wouild change plugs and backed off the timing to 16 and thought i had the problem solved , it ran good for a while but i think the damage was alredy done and it was just a matter of time,

[Mark75H]

I don't think a bad reed block can cause an inline to seize. I cannot believe the motor is turning 6,000+ with the timing at 21° and burning pistons. Most likely it was not really set at 21° and is not now at 16°.

I would recheck the indexing of the crank and sticker.


it was almost like the motor was dropping plugs above 70mph and 6000 rpm

This is typical of over advanced timing.

Propped easy enough to turn 6,400 you should have been able to run 89 with no trouble.

[Raceman]

I don't think a bad reed block can cause an inline to seize. I cannot believe the motor is turning 6,000+ with the timing at 21° and burning pistons. Most likely it was not really set at 21° and is not now at 16°.

I would recheck the indexing of the crank and sticker.

This is typical of over advanced timing.

Propped easy enough to turn 6,400 you should have been able to run 89 with no trouble.

I agree........... don't think there's any chance of it being in the reeds, and I wouldn't put plastic reeds in an inline that was only turning low 6's.

[cannonford57]

any chance a bad bias circuit on a box would make #3 detonate?just a thought

[Mark75H]

I think the first result would be dramatically reduced power and you wouldn't turn 6,000

[Bruster]

Jim,

I think there are some electrical problems. (switch boxs) I would mark the flywheel with a dial indicator on all 6 cylinders and check the timing on all 6. For this test don't worry so much what the timing is, just as long as all 6 fire at the same time.

I'm also thinking that timing will not be the same cranking as running, seems like there was something about that in the service manual.



the timing, that was done by a classmate of mine that is a mercury mechanic it was checked with throttle w/o at cranking with the starter speed we started at 21

[Mark75H]

I think Bruster is right