81 DOHC Sitting for 20 Years

[CB7shwifty]

Hi all,

This is my first post and my first bike. I purchased and 81 CB750C a few months ago and am learning as I go. When I bought the bike the seller was able to get it to run with a spray of starter fluid into the carbs and connecting a battery charger (old, dead battery). I just finished rebuilding the carbs, replacing the needle, needle jet, primary and slow jets, pilot needles, and primary nozzles using and eBay kit.

All of the primary nozzles were stuck and I messed up the first one by stripping the notch so I resorted to drilling them out and using a torx wrench to get a better grip. Two of the pilot screw tips were broken off in the seat. I was able to press them out using a bent dental pick through the tiny hole in the bore. The accelerator pump check valve was also frozen. I was able to free it up with an hour in the ultrasonic with some simple green. I also ditched the SOS.

Anyway, I finally put it back in the bike and got her started! While that was exciting, the idle sucked. It was running at very high rom (around 3k) with the choke fully closed? (I have a hard time with this terminology but the choke was actuated, or pulled out). It would run high for about a minute and then the rpms would start to fall and it would die within 20 seconds or so.

I wasn't able to get it to run without the choke. I have not yet tried to sync the pilot screws yet, but from what I understand it needs to be warmed up and idling without he choke for me to do that.

I'm not quite sure what my next step should be. Messing with the idle adjustment screw didn't seem to help much to get the rpms down. After I had it running for a few minutes, it died and then wouldn't start for several minutes. I had a hard time getting the carbs to seat in the old, hard carb holders. I gave the front of the carbs with PB Blaster Multipurpose, wiggled them in a little further, and was able to start it again with the above mentioned issues.

I would appreciate any insight you all can give to help me get her running smoothly!

Thanks,
Patrick

 

[Chris in Dorset England]

Hi all,

This is my first post and my first bike. I purchased and 81 CB750C a few months ago and am learning as I go. When I bought the bike the seller was able to get it to run with a spray of starter fluid into the carbs and connecting a battery charger (old, dead battery). I just finished rebuilding the carbs, replacing the needle, needle jet, primary and slow jets, pilot needles, and primary nozzles using and eBay kit.

All of the primary nozzles were stuck and I messed up the first one by stripping the notch so I resorted to drilling them out and using a torx wrench to get a better grip. Two of the pilot screw tips were broken off in the seat. I was able to press them out using a bent dental pick through the tiny hole in the bore. The accelerator pump check valve was also frozen. I was able to free it up with an hour in the ultrasonic with some simple green. I also ditched the SOS.

Anyway, I finally put it back in the bike and got her started! While that was exciting, the idle sucked. It was running at very high rom (around 3k) with the choke fully closed? (I have a hard time with this terminology but the choke was actuated, or pulled out). It would run high for about a minute and then the rpms would start to fall and it would die within 20 seconds or so.

I wasn't able to get it to run without the choke. I have not yet tried to sync the pilot screws yet, but from what I understand it needs to be warmed up and idling without he choke for me to do that.

I'm not quite sure what my next step should be. Messing with the idle adjustment screw didn't seem to help much to get the rpms down. After I had it running for a few minutes, it died and then wouldn't start for several minutes. I had a hard time getting the carbs to seat in the old, hard carb holders. I gave the front of the carbs with PB Blaster Multipurpose, wiggled them in a little further, and was able to start it again with the above mentioned issues.

I would appreciate any insight you all can give to help me get her running smoothly!

Thanks,
Patrick

Hi, Forget the carbs for now. You need to find out what condition the engine is in. 1st check compression pressures, ideally 170 PSI, best minimum 140 psi. 2nd check valve clearances, 5 thou MINIMUM, MAXIMUM 7 thou ignore the book. Low pressures need investigating. Most common reason is burnt valves due to undersize clearances. Next stuck piston rings due to lack of use. Sort that lot out first. Keep the stock air box too, do not fit pod filters, down that road there will be big problems. Get a HONDA shop manual, you can find them for free download if you look around on the net.

 

[CB7shwifty]

Hi Chris,

Thanks for the reply!

I did a compression check and all cylinders were equally low (130 +/- 5). This was with a cold engine that hasn't been run is years other than a startup or two a few months prior. After asking around I heard that this compression range seemed reasonable and it should increase once the bike warms up. Your thoughts?

I'll look into the valves and let you know how it goes!

 

[Chris in Dorset England]

Hi,
One way to check for the cause of low compression pressures is, with the plugs out and a cold engine, recheck the pressures, make a note. Then pour in a tea spoon of oil into each cylinder and check the pressures again. If there is no change then the valves or head gasket are suspect, if there's a rise then piston/rings are suspect.

 

[dirtdigger]

Teaspoon of oil is too much for these small displacement motors. That amount will take up a fairly large amount of the combustion space and cause a false higher compression ratio which will show a false high compression pressure. Half that amount at the most in my opinion.

 

[amc49]

X2, give it a minute to drip/run into the ring recesses.

130 dry will run, just not fantastically and it might increase once engine realizes it has to run again and all the rings loosen up in lands.

MILEAGE..................it says how hard the intake rubbers are and no way will it idle right with vacuum leaks there, hard going in of carb bank says they are too hard. They crack doing that then close up once bank is in place but that crack did NOT go away.

You can have carb bank off and do a bench sync by matching all 4 carbs to the first hole the butterflies expose as they open, match to have that first hole just covered to then have all 4 begin to expose EXACTLY the same as the carbs barely open.

Anytime idle mixture screws have broken tips suspect that the seat has been ruined too, it means exact turns of the screws are not exact any longer on the ones with broken tips. The seat/tip interface is very delicate, the amounts of fuel metered there are so miniscule that any scarring or damage of any sort can wildly tilt the pilot output, the seat angle even has an effect, and damaged it is no longer the same. Like drilling jets out only worse.

 

[amc49]

Normal choking is full shut to get it started then open it some and engine will tell you how much, let it run with partially shut until idle begins to race up and then pretty much open it then. You may have to meter it more than that but for sure fully closed only at the start and just beyond for a few seconds. Any more and you wet the plugs out to not start again if it dies, and it WILL if you leave choke fully shut. The DOHCs do not have the little spring loaded windows the earlier fours had that mitigated some of the choke fully closed b-tchiness.

If you are going to make it die because you do not have the choke metering skills then you want it to die from being lean not rich as rich is then much harder to restart. What oh so many people do, then wet plugs and let's foul them out even worse.

Set pilot screws at 2 turns, not having them set it WON'T stay running with no choke. You have made it not so.

 

[CB7shwifty]

Normal choking is full shut to get it started then open it some and engine will tell you how much, let it run with partially shut until idle begins to race up and then pretty much open it then. You may have to meter it more than that but for sure fully closed only at the start and just beyond for a few seconds. Any more and you wet the plugs out to not start again if it dies, and it WILL if you leave choke fully shut. The DOHCs do not have the little spring loaded windows the earlier fours had that mitigated some of the choke fully closed b-tchiness.

If you are going to make it die because you do not have the choke metering skills then you want it to die from being lean not rich as rich is then much harder to restart. What oh so many people do, then wet plugs and let's foul them out even worse.

Set pilot screws at 2 turns, not having them set it WON'T stay running with no choke. You have made it not so.

Hi Amc,

Thanks for the input! The bike has around 11k miles on it and the carb insulators are like hard plastic.

Though I haven't seen any cracks or crumbling, I also suspect a vacuum leak from the carb insulators. Are you familiar with the wintergreen oil softening method? I was considering pulling them and trying it to make them more pliable. Do you think it is worth trying or should I just bite the bullet and spend the $80 on a new set?

That makes sense that the pilot seats might be messed up. Is there anything I can do to repair them or is will I just have to live with them acting squirrelly?

I currently have the screws at 1 1/2 turns out as recommended by my Clymer manual. I'll give them another half turn and see if that helps.

The kid I bought it from messed with the idle adjust screw quite a bit when he was trying to get it running. I don't know how far he adjusted it in either direction. Is there a recommended starting point for this?

Thanks again!

 

[amc49]

I know about it but oil softening of rubber does not occur in minutes, more like a much longer period of time. And if already cracked nothing will be fixing that, the cracking does not simply glue back up.

The seats are what they are, you have no way to fix them, new carb time. The 1 1/2 to 2 comparison is no biggie, either works. Just all the same.

Once you bench sync I will screw the idle speed to just expose the four holes a little bit, likely ends up too high but then while running you screw it back down lower.