Topic: '75 CB400F Bogging under accelleration

[pburress]

Hello, we recently acquired this bike and got it running.  Our mechanic went through it and cleaned/synched/tuned the carbs and did a full tune up including timing, valves adjust, clean and gap plugs etc.  After that we had an issue with a very erratic idle which was resolved by cleaning/lubing the advance and shortening these advance springs a little bit.

Our problem now is, it really bogs down when accelerating from a stop, until it hits about 2500-3000 rpm, at which point it is fine and surges forward with great power.  Our mechanic is quite experienced but is somewhat stumped at this point.  The only thing that might be a clue is, he did say that he was having a problem getting the carbs fully synced with his vacuum gauges which he is normally able to do accurately.  At this point we don't want to spend a lot more money trying things with uncertain outcome.

So, any thoughts on how we can troubleshoot this?

Thanks!

Phil
Wichita, KS

[Tim2005]

Have you still got the stock airbox and filter fittted? Also, what exhaust sytem does it have?

[pburress]

Yes, stock airbox and filter.  The muffler was replaced by P.O. but according to him it has original specs.

Sent from my XT1650 using Tapatalk

[pburress]

Thought I'd give this one bump and also mention that it seems to bog down much worse with more load.  Rode with my wife (no, not a big gal ) on the back and it bogged down much more below about 3k rpm...

[flybox1]

Were the carbs fitted with aftermarket 'rebuild' kits, or were the original brass jets cleaned and reused?
Emulsion tubes under main jets pulled out and cleaned?
Pilot jets turned out and cleaned?
Did replacement float valves get installed?
What size jets are in there now?
What are the air screws set at now?
What do the plug tips look like?   white or carbon/gas/oil fouled?   (a picture for us to see would be good here)

[pburress]

When I got the bike which had sat for several years, I cleaned the tank to squeaky using vinegar and a length of chain(followed by baking soda rinse, water rinse, rubbing alcohol rinse to speed water removal and drying, and finally a rinse with gas/motor oil to give a little oil coat to guard against flash rust.  Then I drained and flushed the carb bowls (nasty green stuff came out) and changed the fuel filter, all before I attempted a first start.   We got it running and rode to my experienced mechanic, who removed and cleaned the carbs, set timing, cleaned and gapped the plugs, set valves, then tuned and synced the carbs (as best he could).  Of course the carb cleaning doesn't rule out some tiny bit clogging a jet or something since then.

We've been riding it since then, dealing with the bogging issue.  I haven't looked at the plugs myself but will check them and get pics when I can.

Question:  If the aftermarket muffler is causing it to run rich, I can drill it out to open it up a little bit.    Any thoughts on this, or do you think this might be the case.

[NobleHops]

Send the carbs out. That's the short and sweet.

[flybox1]

Send the carbs out. That's the short and sweet.

...to someone here. 
You've only got a receipt showing you paid for work....this does nothing to verify the shop did a good job, or even the right job.
If you cant detail to us the work they did... 

Drill out the muffler?  NO.

[hoodellyhoo]

I agree that this sounds more like a carb cleaning issue. That said....

I did actually have a similar issue once and it turned out to be a slide height screw that had worked it's way loose. The engine would bog until a certain RPM and then would accelerate pretty much normally. When I took the gas tank off to investigate it was easy to see that one of the slide height screws was way higher than the other 3. This was also before I had ever synched my carbs so the "chuga chuga chuga" of unbalanced carbs didn't register as a symptom to me. Once I screwed the adjuster back in, all was well. The difference between my scenario and what you're telling us is that my bogging wasn't affected by load.

[pburress]

Thanks for all the good input.  My mechanic is a friend and a good guy with many years experience.  Nobody's perfect of course but I have no doubts that he did the work that he said it did and that he did it well.

Carbs seem to be the most popular guess here.  Since they have not been actually rebuilt, only cleaned, maybe I will get the kits and rebuild them.  I guess I should replace the boots while I'm at it.   This is my chance to spring for a vacuum gauge set!

The plugs have been cleaned and gapped but not replaced recently.  Is it worth a shot to replace them?

Thanks,

Phil

[flybox1]

Thanks for all the good input.  My mechanic is a friend and a good guy with many years experience.  Nobody's perfect of course but I have no doubts that he did the work that he said it did and that he did it well.

Carbs seem to be the most popular guess here.  Since they have not been actually rebuilt, only cleaned, maybe I will get the kits and rebuild them.  Only the soft parts are needed.  Clean and reuse the OEM Keihin jets and needle I guess I should replace the boots while I'm at it.  Only if they are cracked.  Soften them with wintergreen oil.  This is my chance to spring for a vacuum gauge set!  Definitely a worthwhile investment.

The plugs have been cleaned and gapped but not replaced recently.  Is it worth a shot to replace them?  Only if they are really fouled.  A good run on the freeway usually cleans them up

Thanks,

Phil

[pburress]

Never heard about the wintergreen oil thing.  Is it readily available?


EDIT: Never mind, I found and ordered some on Amazon.

[Bodi]

I would try new plugs.
For the carbs, Honda had the carb gasket kits (16010-333-004) the last time I wanted some. This gives you everything to seal up one body plus a set of connector o-rings so four kits gives you an extra set of these.
Aftermarket rebuild kits give you most of the gaskets ... not the connector o-rings. Plus new jets that you don't want to install. Some include a float valve, mixed results with these. It's probably worthwhile to use Viton connector rings anyway, ethanol is not kind to the OEM rubber ones. Someone on the forum was selling Viton ones...
The Keihin float valve set from Honda (16011-329-004) is pricey but they have always worked for me. It is a true PITA to pull and reinstall the carb rack so a reliable float valve is worth the added cost to me!
My Morgan Carbtune synchronizer has been trouble free for many years. A gauge set will do just fine as well, I chose the Morgan as it's compact and relatively indestructible.

[pburress]

Thanks Bodi.  I guess for less than $10 new plugs won't hurt!  I can keep the old as spares and keep one in the bike's tool kit.

[flybox1]

Never heard about the wintergreen oil thing.  Is it readily available?


EDIT: Never mind, I found and ordered some on Amazon.

This is the cheapest ive found.
https://www.amazon.com/Tenda-Horse-Products-OIL-WINTERGREEN/dp/B00562VC04

2-3 TBSP in double ziploc baggies and toss in your carb boots.  Shake n rotate a few times a day.
No boiling or acetone needed.  48hrs later they are supple.  Wear gloves.

[pburress]

I'm picking this thread up again since the problem remains.  I've replaced the plugs, coils/wires and completely rebuilt/synced the carbs.  Still, it bogs down and hesitates while accelerating when its under 3k rpm.  At 3k and above it is great. 

I had the compression checked and it's not ideal but I'm not sure how bad this is:
#1  120 lbs
#2  118 lbs
#3  120 lbs
#4  130 lbs

My mechanic has also cleaned the points and set the gap, set timing, and adjusted the valves.

I'm at a loss!  Any thought for thinks I can check next?

[strynboen]

 ihave seen som cams vho are veared dovn..you can see the Sharp egdes..hvere the lifter runs on the cam..and often there stands a shart egde hvere the lifter not veares ..and that gives a good vear indikator.

.have yoy got the valve cover off the bike??

have you thekked the breather tube..try to take it of ythe head and thek for blow dovn pressure..it komes aut at the top..a bad engine bloves a damp..a good engine dont give anuthing aut there...i have thanged pistons and cylenders..and all blow aut is gone .

[Bodi]

Confirm the spark advancer is working properly. Sorry if that's already been discussed. Late advance operation can do just what you describe.

[pburress]

We've messed with the spark advancer twice.  One of the times we shortened the springs a little bit.   

[evinrude7]

any chance you have the wrong size main jets?  mine bogs a little until it really gets going.  recently found out i have 115s in there when it should have 110s. 

[Lou]

I'm having this same problem ... my bike - bogs when I let the clutch out to take off from a stop, less that 3k rpm. Other than that it runs great.

Can anyone tell me where is the "slide height screw"? Earlier in this thread, this was the culprit for someone else so I figure I'll check it out. Here's the quote from earlier thread I'm referring to:

I did actually have a similar issue once and it turned out to be a slide height screw that had worked it's way loose. The engine would bog until a certain RPM and then would accelerate pretty much normally. When I took the gas tank off to investigate it was easy to see that one of the slide height screws was way higher than the other 3. This was also before I had ever synched my carbs so the "chuga chuga chuga" of unbalanced carbs didn't register as a symptom to me. Once I screwed the adjuster back in, all was well. The difference between my scenario and what you're telling us is that my bogging wasn't affected by load.

[Redline it]

should be right between 1/2 and 3/4,  there might be a little chrome or stainless strap connecting the pairs, 4 locknutted adjuster screws. 

[Deltarider]

Provided ignition in all its aspects is fine and you have OEM carb parts and air filtration, you could experiment with adjusting the airscrews or even an other size slow jet. The Honda CB400F Partslist lists no less than three different sizes, knowing #38, #40 and #42. Cf. p.16 http://www.honda4fun.com/pdf_parts_list/pdf_spac400/CB400F-F1-F2/CB400F-F1-F2.pdf

[scott jones]

Good morning. Scott here. I have a similar problem with my 1973 cb500. Seems to bog off idle under load but goes like a raped ape after about 3 grand. I rebuilt the carbs this past summer with aftermarket kits. I recently put all the original brass back in except the slow jets. I had 1 slow jet broken in carb #1 but I was able to get the little broken tip of the jet out of the carb. With that being said I stayed with the aftermarket slow jets. Does anyone think this bogging is ignition related? BTW  Stock carb adjustments, Stock air box but aftermarket baffled 4 into 1 exhaust.

[Herr Supersport]

Boys girls and others

Same bike, same issue. Since ever.
Bought the bike in 1986 / 20'000 km and it's shure that nobody did any tinker before.

I finally solved the problem by give it 2° more ignition-advance.

Give it a try.
It will cost you 5 minutes and is 100% reversible.

Background informations:
Just to be shure the ignition does what it has to do I installed a fully programmable accent-igni. Prob remained, means it was carb-related.
2° more advance will raise RPM. To reset idle at 1200, you have to lower the slides, what obviously improves transition.
And: Keyster stuff made me nearly nuts. After installing all Keihin brass in original dimensions, the bike now runs perfect.

Much luck!
Daniel