Perplexing problem - ignition vs. carburetors

[nargent]

Hello all,

I bought a nicely kept 1979 CBX the year before last. It was always difficult to start, but since I bought it in November, just before temperatures fall below -20C in Canada, I didn't think too much of it, as I wasn't going to be able to do much before spring of last year.

The bike had been 'restored' by the previous owner, and cosmetically was in beautiful condition. The obvious maintenance items (drive chain and sprockets, brake pads etc) were new, and everything was clean and shiny. The seller said that the engine had been stripped down and rebuilt, and the carburetors also stripped and rebuilt. I couldn't see anything externally to suggest anything had been butchered.

The bike remained difficult to start, and often felt like it was dropping a cylinder or two at times. It was not noticeably worse either hot or cold - just intermittent. By the end of summer I thought I had pinned it down to cylinders 1 and 6, but using a temperature 'gun' cylinders 1 and 6 were much cooler, while 2 and 4 were cooler than 3 and 5 (ie, NOT 2/5 vs. 3/4). Bear with me here - I'm getting there...!

I figured it was likely the igniters, but thought that the easiest route would be to replace the entire ignition system, so I installed a Sachse system from Italy. By the time it arrived and got installed, it was December and snowing heavily, so the bike was put to bed for the winter again.

Back to 2023 - I couldn't get it to start at all, but confirmed the ignition was at least making sparks. I had been reading some horror stories of gummed up carburetors, and wondered if the so-called 'rebuild' actually included the carbs, so, I got a set of Keyster kits and stripped and rebuilt the carburetors myself. I was hoping to find all kinds of gunk blocking jets etc. but the carbs were actually very clean looking, and no sign of any bad handywork was apparent. Jetting and needles were correct for stock. I got the carbs back on the bike last week, and got the bike to run - but without the assistance of cylinders 1 and 6. Even though I could see healthy sparks from the plugs on all cyclinders, I ordered some new coils, and replaced the 1 and 6 coil. Result - exactly the same - will fire up 2,3,4 and 5 but cylinders 1 and 6 are not getting hot!

I am kind of struggling to figure out what could be wrong at this point. I have trimmed the ends of the ignition leads and refitted them with new plug caps. Still seeing sparks at 1 and 6, but no actual ignition. The plugs appear to be getting fuel. Is there something obvious that I am missing here? Any and all suggestions would be appreciated.

[CBX-tras]

Keyster kits are the worst. If you replaced any of the original metal piece with their brass ones, you're doomed. All junk.

The idle (pilot) jets can be compromised in just 2 weeks if the carbs were left with fuel in them. If those jets weren't extracted to clean the jet and the circuits it feeds, it's all for nothing.

Additionally, if you have stock coils, those are problematic too. MP08 coils from newer Honda's are better (epoxy filled) and install with minor modifications.

P.S. I hope you didn't trash the unobtainable 05F slide needles from the carbs. It'll cost hundreds to replace them, IF you can find them for sale.
My carburetor seminars at the Barber this fall with specifically address CBX carburetors with valuable tricks and tips and do's and don'ts.

[Larry Zimmer]

JR -- tells us more about your coming seminars at Barber. Good stuff.

[Crazy Canuck]

How about compression tests on 1 and 6....maybe valves are mis-adjusted and hanging open.

Another thing to try.....I was able to get a small fuel line directly into my #6 intake through the filter box and applied short squirts of fuel from a syringe and was able to confirm that once the cylinder gets fuel it fires and heats the exhaust pipe...confirmed with a temp gun.

[CBX-tras]

I will be giving 2 seminars at Barber with my associate Kurt Muehlbacher of KM Carburetor. One each day, October 6th and 7th.

We will be focusing on CV carburetors, specifically the CBX, showing tips and tricks and headaches to avoid. Our theory is that if you can rebuild a rack of CBX carbs, you should have the skillset to do almost any.

I'll even be demonstrating my method of fast and easy pilot jet extraction.

Kurt will be providing hand-outs, samples and a few door prizes from his ever-expanding carburetor parts and JIS tools offerings.

The seminars are free to attend once you're on the grounds.

Location and times are TBD but, there could last up to 3 hours each, depending on attendance and the Q&A from the participances.

We hope to see a lot of folks there.

[Larry Zimmer]

Sounds great, JR. Thanks! Hopefully, I can get there this year.

[nargent]

Thanks CBX-tras and others. I appreciate the input.

I have heard good and bad things about all the manufactured carb kits. I have used keyster parts for two Honda CB400fs without problems over a number of years, but your point is well taken. For what its worth, I didn't replace the original needles, as the originals still looked pristine. In fact the only things I did replace were the o-rings and floats (the latter only because I dropped one on the floof and couldn't find it - so all were replaced...) I did remove and blow out all the jets and the idle assembly, but as I mentione, I was surprised (and a bit disappointed!) to find that everything looked in very good shape.

I hadn't thought to check the compression - thanks for that - but all cylinders are close to 110 psi with no significant difference evident at 1 and 6. I also went back and checked the pickup for 1 and 6 on the Sachse system, but all loolks good there too.

So its still a puzzle. I am out of action for a few weeks as I have just had a new joint put in the base of one of my thumbs (the other one will be done in winter), so I am going to leave things alone and hope that the bike has healed itself in the meantime. Sometimes doing nothing is the best therapy.

Thanks again. Nick

[nargent]

Oops - I meant 130 psi - my bad!