Motorcycle

Motorcycle questions? Ask a mechanic.

Connect one-on-one with {0} who will answer your question

This answer was rated:

I have restored a 1981 CB900F. Cylinders 1,2 and 3 run grey…

I have restored a...

I have restored a 1981 CB900F. Cylinders 1,2 and 3 run grey but 4 runs black like the choke is on. Carbs are not flooding, floats same, new float needles, timing correct, shims good. Tried another HT coil in the event it was electrical and runs just the same. Any ideas?

Mechanic's Assistant: Do you know your ignition timing number? How many miles are on the bike?

Sorry what do you mean by 1gnition timing number? Timing is all correct. Mileage about 60,000

Mechanic's Assistant: What is the make/model of your '81 bike?

Honda CB900FB

Mechanic's Assistant: Are you fixing your bike yourself? What have you tried so far?

Yes, I am. I used to be a bike mechanic myself. That is the frustrating bit. My thoughts are now to replace Carburettor number 4. As I say all runs well except cylinder 4 as black as hell. Floats and needles are new, not flooding, timing correct. 1 and 4 and 2 and 3 run in those groups. Tried another HT coil but no change.

Mechanic's Assistant: Is there anything else the Mechanic should know before I connect you? Rest assured that they'll be able to help you.

Think that is about it.

Show More
Show Less
Ask Your Own Motorcycle Question
Customer reply replied 1 year ago
Posted by JustAnswer at customer's request) Hello. I would like to request the following Expert Service(s) from you: Live Phone Call.Let me know if you need more information, or send me the service offer(s) so we can proceed.
Answered in 6 hours by:
7/6/2021
Motorcycle Mechanic: TIMOTO, Motorcycle Mechanic replied 1 year ago
TIMOTO
TIMOTO, Motorcycle Mechanic
Category: Motorcycle
Satisfied Customers: 1,624
Experience: Honda registered technician, silver level award, former Kawasaki/Suzuki tech, 40+ years exp.
Verified

On a bike that old that has probably had the carburetors worked on many times in the past it could be very difficult to locate the problem.

1. With the engine warmed up try turning the idle mixture screw on the bottom of the #4 carb in and see if it makes any difference in the idle speed. If the idle speed increases then the problem is in the idle circuit. Completely remove the idle mixture screw and closely examine the taper on the screw. If the tip is bent then get a new screw and try adjusting the mixture screw again. All of the mixture screws should be adjusted individually to produce the highest idle speed on each cylinder. They should all end up being within 1/4 turn of each other. If #4 is still running rich even with the mixture screw turned all of the way in and if the tip on the old mixture screw was bent then the carburetor body where the taper of the screw fits is damaged and not repairable, it will require a new carburetor.

2. Try swapping the #4 slow speed fuel jet with the #3. If the rich condition moves to the #3 cylinder then the #4 jet is the problem, most likely it was drilled out in a previous attempt to clean the carbs.

3. Inspect the air cut valve on the side of the carburetor body looking for any damage to the rubber diaphragm.

4. Swap the slide , spring and needle from #4 with #3 to see if the problem moves to the other cylinder.

5. Totally disassemble and clean the #4 carburetor body by soaking it overnight in a commercial grade of carburetor cleaner. Rinse and blow dry the carb then verify that the low speed air bleed circuit is not clogged by spraying some aerosol carburetor cleaner thru it.

TIMOTO
TIMOTO, Motorcycle Mechanic
Category: Motorcycle
Satisfied Customers: 1,624
Experience: Honda registered technician, silver level award, former Kawasaki/Suzuki tech, 40+ years exp.
Verified
TIMOTO and 87 other Motorcycle Specialists are ready to help you
Ask your own question now
Was this answer helpful?

How JustAnswer works

step-image
Describe your issueThe assistant will guide you
step-image
Chat 1:1 with a motorcycle mechanicLicensed Experts are available 24/7
step-image
100% satisfaction guaranteeGet all the answers you need
Ask TIMOTO Your Own Question
TIMOTO
TIMOTO
TIMOTO, Motorcycle Mechanic
Category: Motorcycle
Satisfied Customers: 1,624
1,624 Satisfied Customers
Experience: Honda registered technician, silver level award, former Kawasaki/Suzuki tech, 40+ years exp.

TIMOTO is online now

Related Motorcycle Questions
My virago 750. Carburettor float stayed open and filled the
My virago 750. Carburettor float stayed open and filled the crankcase with fuel. I drained 6ltrs, fitted new oil filter cleaned carburettors rear cylinder is perfect but the front is making the plug s… read more
kwiggins29059
kwiggins29059
Motorcycle Mechanic
Associate Degree
412 satisfied customers
I have a 1999 Polaris Sportsman 500 I put a new carb and
I have a 1999 Polaris Sportsman 500 I put a new carb and weight t keeps flooding it won't start I checked the gap on the valves they where out a little bit not now still won't start and the cylinder i… read more
Mason Surratt
Mason Surratt
Mechanic
Vocational, Technical or Trade School
306 satisfied customers
Suzuki motorcycle GS850G: Motorcycle: 1980 GS850G.I'm experienci
Motorcycle: 1980 GS850G. I'm experiencing a significant loss of power that seems to be getting worse, and I have fuel pooling in the air box when the engine runs. Carbs have been rebuilt and the float… read more
Wayne S.
Wayne S.
Motorcycle Mechanic
Fully Licensed Motorcycle Mechanic
1,435 satisfied customers
2001 250 cn helix motor good fuel enrichment valve good elect.
2001 250 cn helix motor good fuel enrichment valve good elect. fuelpump good. comp. good. starts won,t stay running no advanceing of rpm, wet spark plug… read more
Wayne S.
Wayne S.
Motorcycle Mechanic
Fully Licensed Motorcycle Mechanic
1,435 satisfied customers
2005 Honda XT750CB with black smoke flooding condition after
2005 Honda XT750CB with black smoke flooding condition after up to operating temp, when cold runs good, after coming to operating temp, black smoke, loss of power, ambient temp 70 degrees. Has new car… read more
Christopher
Christopher
Senior Mechanic & Technician
Factory Training
2,435 satisfied customers
Hi I have a Yamaha 1983 XJ 900. I recently overhauled the carburettor&
Hi I have a Yamaha 1983 XJ 900. I recently overhauled the carburettor's as the bike was not in running order at time of purchase. I have set and checked the carburettor's as per the specifications and… read more
Louie
Louie
Shop Owner
1,982 satisfied customers
I have a 95 artic cat puma 340 f/c with no spark all the switches
I have a 95 artic cat puma 340 f/c with no spark all the switches seem to be working I get about 90 volts out of the cdi when pulling it over is that enough ? the coil was replaced, I'm thinking it wa… read more
Doug E.
Doug E.
Vintage M/C restorer
Vocational, Technical or Trade Scho
2,485 satisfied customers
3 cylinders..firing..ht leads so that 1-4 have run on the coil system
1981 cb900f 2-3 cylinders are not firing, pulsers have been tested and checked, igniters have been tested and checked, coils have been tested and checked. Coil wires have been changed over as well as … read more
TIMOTO
TIMOTO
Motorcycle Mechanic
Bachelor's Degree
1,624 satisfied customers
Runs good..next time..backfires misses wont accelerate leave it sit
runs good one day next time starts hard backfires misses wont accelerate leave it sit for a couple hours starts and runs good… read more
TIMOTO
TIMOTO
Motorcycle Mechanic
Bachelor's Degree
1,624 satisfied customers
1&4 cylinders are not firing on my 1981 cb900f, It..smoothly..Wiring
1&4 cylinders are not firing on my 1981 cb900f, It has spark and fuel the spark igniters have been tested . 2&3 fire and run smoothly. Wiring has been tested for short circuits etc. Can any one sugges… read more
KKassel
KKassel
OWNER
High School or GED
52 satisfied customers
1986 kx 80 starts up on a bump but have very low revs no power
i have 1986 kx 80 starts up on a bump but have very low revs no power but when its out of gear i have revs how do i fix this or what is the problem… read more
.......
.......
Vocational, Technical or Trade Scho
211 satisfied customers
My gsf 600 bandit is running lumpy and stalling when you put the power
my gsf 600 bandit is running lumpy and stalling when you put the power on what might it be… read more
MrHonda
MrHonda
Vocational, Technical or Trade Scho
398 satisfied customers
bad fule line diaphram (leaking gass when engine running) ande..bypass
hi - I have a 1982 CB900F with a bad Fule Line Diaphram (leaking gass when engine running) ande I can not find a replacement part. Can I bypass this valve or any suggestions on finding a replacement p… read more
MrHonda
MrHonda
Vocational, Technical or Trade Scho
398 satisfied customers
1975 gl: rebuilt..my bike keeps flooding out the plugs loading..seams
mr. honda, i have a 1975 gl 1000, iam having carb problems i have had my carbs rebuilt twice at a cost of about 700.00$ so far it has not worked the bike sat for a long time so everything was was gume… read more
MrHonda
MrHonda
Vocational, Technical or Trade Scho
398 satisfied customers
REBUILT..REV IT OUT IT BOGS DOWN AND DIES, AS IF ITS FLOODING..CDI
HI, I HAVE A 1979 XR 250 THAT I HAVE REBUILT, I HAVE GOT IT RUNNING RESONABLE, BUT WHEN I GO TO REV IT OUT IT BOGS DOWN AND DIES, AS IF ITS FLOODING, I HAVE CLEANED THE CARB AND CHECKED THE SETTINGS B… read more
Graham Percival
Graham Percival
Manager
Vocational, Technical or Trade Scho
94 satisfied customers
1981 suzuki: cylinders..seats..the coils, spark plugs, plug wires
I have a 1981 suzuki gs650g, it is only running on half its cylinders. I cleaned the carb including needles and seats, changed the coils, spark plugs, plug wires, and nothing has changed. what else sh… read more
Doug E.
Doug E.
Vintage M/C restorer
Vocational, Technical or Trade Scho
2,485 satisfied customers
Idle screw s out 3 turns , no shims..factory..exhaust..pipe
got a 93 fzr 600r yamaha with a flooding problem , set float levels to 24 mm ...new needles , idle screw s out 3 turns , no shims under slide needles ,factory stock jetting , ss2r stainless exhaust ca… read more
Christopher
Christopher
Senior Mechanic & Technician
Factory Training
2,435 satisfied customers
What will cause carbs to flood and expell gas from vent ...
What will cause carbs to flood and expell gas from vent tube in the Honda VT1100 engine? Have already completely rebuilt carbs and replaced floats. Carbs are adjusted properly. Bike has 50K miles. Is … read more
Motorcycle Tech
Motorcycle Tech
1,297 satisfied customers
Disclaimer: Information in questions, answers, and other posts on this site ("Posts") comes from individual users, not JustAnswer; JustAnswer is not responsible for Posts. Posts are for general information, are not intended to substitute for informed professional advice (medical, legal, veterinary, financial, etc.), or to establish a professional-client relationship. The site and services are provided "as is" with no warranty or representations by JustAnswer regarding the qualifications of Experts. To see what credentials have been verified by a third-party service, please click on the "Verified" symbol in some Experts' profiles. JustAnswer is not intended or designed for EMERGENCY questions which should be directed immediately by telephone or in-person to qualified professionals.

JustAnswer in the News:

Ask-a-doc Web sites: If you've got a quick question, you can try to get an answer from sites that say they have various specialists on hand to give quick answers... Justanswer.com.
JustAnswer.com...has seen a spike since October in legal questions from readers about layoffs, unemployment and severance.
Web sites like justanswer.com/legal
...leave nothing to chance.
Traffic on JustAnswer rose 14 percent...and had nearly 400,000 page views in 30 days...inquiries related to stress, high blood pressure, drinking and heart pain jumped 33 percent.
Tory Johnson, GMA Workplace Contributor, discusses work-from-home jobs, such as JustAnswer in which verified Experts answer people’s questions.
I will tell you that...the things you have to go through to be an Expert are quite rigorous.

What Customers are Saying:

I would (and have) recommend your site to others I was quite satisfied with the quality of the information received, the professional with whom I interacted, and the quick response time. Thanks, and be sure that I'll be back whenever I need a question answered in a hurry.

Stephanie PElm City, NC

used your service this weekend with "Trecers" help. thank you ,thank you, thank you. replaced an A/C fan motor. Local Auto Zone had part. $15.00 "tracer" fee and $40.00 for parts, I saved several hundreds of dollers at a shop. i will recommend you and use you in the future.

David L.Richmond, TX

9 dollars, 2 hours of my time, and I drove away. Your diagnosis was right on the mark. Thank you so much.

PhilMarysville, CA

Lurch. Thank you very much. I had real doubts about this website but your promptness of response, quick followup and to the point answer with picture was incredible.

CharlesWalnut Creek, CA

As a single woman, I really appreciate an excellent and affordable opinion.
Thank you Geordie, I will not hesitate to contact justanswer in the future!

SueCharleston, WV

Another great insight to what may be the problem. I will have my mechanic take a look at it tomorrow. Thanks again, Frank...you do indeed know your stuff.

JimCastleberry, FL

Wonderful service, prompt, efficient, and accurate. Couldn't have asked for more. I cannot thank you enough for your help.

Mary C.Freshfield, Liverpool, UK

< Previous | Next >

Meet the Experts:

TIMOTO

TIMOTO

Motorcycle Mechanic

1,624 satisfied customers

Honda registered technician, silver level award, former Kawasaki/Suzuki tech, 40+ years exp.

Andre Pilon

Andre Pilon

Motorcycle Mechanic

1,055 satisfied customers

Over 25 years experience, ICS, MMI, Owned shop for 12 years, worked tech-support for Harley Davidson

asphaltcycles

asphaltcycles

Motorcycle Mechanic

468 satisfied customers

I operate a motorcycle dealership and CCS Ducati Race team.

kwiggins29059

kwiggins29059

Motorcycle Mechanic

412 satisfied customers

Experience in Harley Davidson and Triumph

Glenn Adams II

Glenn Adams II

Motorcycle/ATV Technician

393 satisfied customers

6 years Motorcycle/ATV/Water Craft; 4 years auto; 18 years carpenter

Wayne-Mc

Wayne-Mc

Motorcycle Mechanic

293 satisfied customers

31 Years experience with Harley-Davidson motorcycles, licensed Motorcycle Mechanic, Shop Owner

Terry

Terry

Motorcycle Mechanic

198 satisfied customers

Over 20 years experiance multi-line tech specializing in H-D. troubleshooting and advanced fuel injection

< Previous | Next >

Disclaimer: Information in questions, answers, and other posts on this site ("Posts") comes from individual users, not JustAnswer; JustAnswer is not responsible for Posts. Posts are for general information, are not intended to substitute for informed professional advice (medical, legal, veterinary, financial, etc.), or to establish a professional-client relationship. The site and services are provided "as is" with no warranty or representations by JustAnswer regarding the qualifications of Experts. To see what credentials have been verified by a third-party service, please click on the "Verified" symbol in some Experts' profiles. JustAnswer is not intended or designed for EMERGENCY questions which should be directed immediately by telephone or in-person to qualified professionals.

Show MoreShow Less

Ask Your Question

x