79 johnson 100TLR79S 100 HP compression spark & fuel

[ceddon46]

I have just got through with a cylinder repair honed out , new piston / rings / bearing,s , this is #3 cylinder after I put back together and started it up after warm up I did a compression check and the 3 old cylinders are still 125psi and new cylinder is 110 psi kinda strange for it to be lower being new and all ( but maybe rings have not seeted as of yet with only about 30 min on engine ) but I am having an issue with the new cylinder not pulling its load has good spark and compression but when I pull plug wire no change in rpm pull one above it and engine dies so tried the other 2 cylinders and each one rpm changed but engine would not die , so this tells me that the one cylinder above the one repaired is pulling all the load for that side since engine died when I pulled plug wire on cylinder #1 above #3 which was one I repaired , so my question is since that cylinder has compression and spark could it be getting to much fuel are not enough ( I rebuilt both carbs 2 weeks before engine went down ) what is best way to check and make sure that cylinder is getting fuel ? are am i thinking way off base here and it be something else.. oh and new coils and plugs ( tried both gap .030 and .040 ) 40 seems to idle better

 

[ceddon46]

Re: 79 johnson 100TLR79S 100 HP compression spark & fuel

I am thinking maybe not enough fuel to that cylinder might be what caused it to blow a ring in the first place .. I dont know what i could have done wrong on the carb rebuild cause they are so easy fixed jets and all ..

 

[emdsapmgr]

Re: 79 johnson 100TLR79S 100 HP compression spark & fuel

You are probably on the right track. Always best to determine just what caused the failure in the first place, so it won't repeat on a new powerhead. When a cyl goes bad, the carb is usually a prime suspect for the failure. It should have been completely torn down, including the jets. Esp the main jet in the bottom of the bowl. Possible restriction in that main jet that feeds #3 hole. If you've got good spark on 3, but no effect when you pull the #3 plug wire, it's probably not getting fuel. In the same regard, if it's not getting fuel-it is also not getting any oil. Hopefully it's not a reason for the low compression. The 79 carbs don't have mid-jets, just idles. You'll want to pull all the jets (mains and idles) out of all barrels and visually check them against a bright light. See what you find. 125 is normal compression for that engine when new. If you have any original fuel hoses on that engine-they are not rated for today's ethanol fuels. I've seen black bits of fuel hose in the jets.

 

[ceddon46]

Re: 79 johnson 100TLR79S 100 HP compression spark & fuel

ok I will pull the jets did not do that when rebuilt the carbs just did what was in the kit cleaned and blowed them out good , just talked to a mechanic her were I live he told me that the original piston for this engine were faulty with the rings being to close to the top of the piston and with the new fuel ( ethanol ) would burn up the original piston , and thats why they carried the 125 PSI compression, so he asked me If i used sierra are another brand piston which I did and he said thats why compression was lower not to worry about it , but he also said to put 3 sizes smaller main jets in and to back down the WOT screw two turns and this would help in burning the ethanol fuel in this engine for it to last .. This sound right to you guys ?

 

[ceddon46]

Re: 79 johnson 100TLR79S 100 HP compression spark & fuel

Doe's it take a special toll to remove the jets are just a screw driver ?

 

[emdsapmgr]

Re: 79 johnson 100TLR79S 100 HP compression spark & fuel

Bombardier makes a special tool which you can order from any dealer: 317002 Works on all their carb jets. Not sure why you'd go smaller with the jets, that means less fuel (and lube oil) while running. You'd wind up with a lean condition for sure. Leave the jets original/factory. If you are concerned about long-term durability, retard the timing slightly.