Cyl 1 and 3 cold on 82 CB750sc - diagnostic help

[lockeed]

Hey Guys, first time poster here.

Last summer, I bought this 82 Nighthawk that had been stored for the past 3 years.

Just started my Café build but I need to have the bike inspected to plate it. Bike didn't wanted to start so I started by removing the carbs and cleaned them using the procedure found on the forum. I let the whole stack soak in Pinesol overnight etc... Took extra care in cleaning the internals etc...

Put the carbs back, drained the tank and put fresh gaz in, bike started right away but something was wrong. Could barely keep turning. Tried to diagnose and found that it's running on cyl #2 and 4 only. Cyl 1 and 3 are cold to the touch (exhaust).

Since cyl 2 and 4 don't use the same coil, at least I know that the coils aren't the issue. So I inspected the wires, all looks good. Unfortunately, I don't have new spark plugs at the moment but the one I do have looks pretty good and they seem to fire fine as they don't get wet, telling me that it might be a fuel related issue more than a spark related one.

Tomorrow morning, I'm buying new plugs and putting them in just to be on the safe side. Tonight, I pulled the carb bowl from carb #1 and checked the air vents and re-cleaned the jets... No success but I found that the exhaust would become slightly warm to the touch afterwards, which it didn't do this morning...

Any tips is welcomed.

 

[amc49]

Depending on how diluted the Pinesol was you may have ruined the carbs. That stuff will eat away aluminum in as little as 15 minutes at straight out of the bottle. Hope you removed 100% of all the rubber before dunking parts but you didn't if you did not remove the carbs from each other.

If bike not running on 1 and 3 to begin with was the reason for storing it..................well, you see the problem there.

 

[lockeed]

I'm afraid that some rubber spent an entire night soaking. Per instructions, I didn't remove the air cutoffs.

I'm part to blame as well, I didn't realized Pinesol was that hard on aluminium and rubber. That might be the culprit.

 

[amc49]

I've seen it eat pits in aluminum like pistons before straight and in only 6-8 hrs when playing with it. It removes like cad plating in an hour or less and paint used over polished aluminum to etch the aluminum past the polish in 15 minutes.

Kiss the rubber parts goodbye. May well have altered the slide to carb top cover clearance that lets the slides work too, hope not. That clearance is critical to making slides work.

I've used other cleaners that didn't really do spit either and was freaked out at how fast Pinesol worked, it's like the oldschool Safety-Kleen carb dunk for industrial applications we used in the garage back in the '70s, some kick-ss powerful stuff that ruined parts in exactly the same way. They removed it off the market for pollution concerns from what I heard, I looked for a duplicate of it for years until finding the Pinesol. Still not sure exactly what in it does that, the ingredients in the MSDS doesn't seem that bad unless it has something to do with the pine oil thing.

I use it full strength at 10 minutes max. It's the rage for super cheap carb cleaner but nowhere have I seen any cautions as to how powerful it is. Thankfully I started out with only a very short test and didn't ruin anything myself, I almost did.

 

[lockeed]

Wow, wish I had knew... I'll order a rebuild kit I guess.

Any of you guys replaced the VB42 for mikunis?

 

[amc49]

Most go to race Keihins.

 

[lockeed]

Thanks for the info AMC - I'll look them up. To your knowledge, are the race Keihins more suited for pod filters?

Back on my carb, spent most of the day taking them appart, but really appart this time per the manual found here, rev G. Long story short, they weren't as clean as I thought!

Putting them back on the bike tomorrow.

Looks like the Pinesol damaged my small rubber diaphragm on 3 out of 4 carbs, but it didn't eat through them completely, just the first layer of rubber it seems. I cleaned them out the reinstalled them as I'm still waiting for the rebuild kit... They should still work though. Lesson learned.

 

[amc49]

'...they weren't as clean as I thought!'

Common and many go through them more than twice.

Virtually any carbs can use pods except for CV types, which rely on the airbox restriction to open the slides fully in varying amounts depending on the slide type.

One could use Mikunis but need a lot of things along with carbs like manifolds, throttle setup, and a suitable pre-jet regimen so engine doesn't fry trying to get them dialed in.

 

[lockeed]

Let's take a CR31 sidedraft racing keihin, is it still considered a constant velocity type?

 

[amc49]

No.

CV carbs do not have a direct cable connection to the slide. The CRs use cables and are direct lift carbs. Meaning the slides move anytime you move the throttle. CVs do not, you are opening a butterfly, but the slide opens 100% by itself and can lag your opening the butterfly by hand.

 

[lockeed]

Thanks for that info. Bike runs like a champ now btw

But since the bike will be converted into a café racer, my plan is to use pod filters.... Or, I've seen some use a small airbox where all the carburetors are hooked to a single large pod. Wonder if this could work with the stock carbs....?

 

[amc49]

If the big filter is not restrictive enough the carbs will still suffer. The paper filter used in OEM airbox runs better than say a K&N stock replacement, which flows free enough to make some of the pod issues.

 

[lockeed]

So one might try to restrict some of the airflow on a larger pod like that and see what's what... Interesting.

 

[amc49]

If only it was as simple as that.........................you put on a big filter to then ruin it intentionally by taping it up or other rigs. It has lead to some pretty comical looking bikes.

Like so many others you may be learning CVs and pods are often not a good combo. Everybody is a scientist nowadays.

Look here.................use 4 individual pods and stock CVs.................the motor MUST be in jam-up shape (valves and ring seal, compression test will tell you) or a waste of time................need header to help it work better..........primary jet 75 and secondary 115 and it will run great, I did it for years. If the motor is in not as good a shape as so many think you will choke and give up.

Motor will run fine but no better than it did before, the pods actually offset a bit of power that the header makes and bike will not quite pull out max rpm in high gear like it did before because carbs not quite opening up fully with the pods. You won't care because you will still being running at 110 mph+.

A lot of people can like that.

You are in a northern clime, I noted back when everybody was doing this that up north there were LOTS more complaints about not being able to get bike running right with pods. Here in the south they seem to run fine. May be worthless observation but there nonetheless.

 

[lockeed]

Again, thanks for that valuable information.

I'm planning on taking the engine out anyway to do at least a top end check, valve adjustment, rings, pistons, cylinder, etc... Would you systematically replace the rings/seals on it even though the bike only as about 20K miles? Assuming that visually, everything looks good of course.

 

[amc49]

Go by bore condition, Honda rings at least on the early DOHCs do not wear that fast unless bike abused. Long term sitting can rust pit the walls though depending on the sit conditions.

Normally no, the exhaust valves will be the worst offender and possibly some intakes. For sure change the valve seals. Valves change only, they CANNOT be recut.



20K miles and rings are commonly OK.

 

[lockeed]

Glad to hear that. Thanks again for the info.