Acceleration issue

If it's broken or just needs tweaked

Moderators: Volker_P, tidd650

User avatar
Rocket Man
Posts: 27
Joined: Sun Aug 11, 2013 11:17 am

Acceleration issue

Postby Rocket Man » Mon Sep 16, 2013 2:34 pm

I’m restoring a 1980 CB650C with 7000 miles on it and have done the basic tune up and carburetor work and still have an issue. I could use the voice of experience here, so I’ll describe the situation as best I can. She starts up and runs fine on the choke, with a little hesitation as the throttle opens. As she warms up, never getting out of 3rd gear up and down the street, I can reduce the choke to maintain around 1100 rpm. Acceleration is not ideal from a closed throttle and when the choke is all the way off it hesitates or dies when the throttle is opened. Starts right back up with no throttle so first suspect would be the accelerator pump, but it looked and acted fine when apart. Had some issues with spark plug leads until all connections were tested (with meter) and corrected. I have not adjusted the idle mixture screws yet and the plugs look ok. Could weak igniters or timing play a big part in this? Igniters show signs of heating up and timing was set the static way. The slow idle jets are not plugged, but could be partially plugged. I ran a copper wire through. What should I investigate next or visit again?

Thanks

MiGhost
Posts: 379
Joined: Tue Jun 07, 2011 2:30 am
Location: South Central Mitten

Re: Acceleration issue

Postby MiGhost » Mon Sep 16, 2013 6:58 pm

Rocket Man wrote:The slow idle jets are not plugged, but could be partially plugged. I ran a copper wire through. What should I investigate next or visit again?


The slow/idle jets need to be pulled out to clean them properly.

Ghost
~ Ghost

PD50 Carb Info
82 CB650SC Brake Caliper Info

Stable
Wayward Son:1980 CB650C, RestoMod Period Custom Touring
Bad Moon: 1983 XJ750MK Midnight Maxim, Full Dress Tour Deluxe w/ X-1 Fairing
Nemesis: 1983 XJ750K Maxim, Old School style chopper

User avatar
Rocket Man
Posts: 27
Joined: Sun Aug 11, 2013 11:17 am

Re: Acceleration issue

Postby Rocket Man » Mon Sep 16, 2013 9:41 pm

Say it ain't so! No amount of seafoam will clear those pressed in jets over time?

MiGhost
Posts: 379
Joined: Tue Jun 07, 2011 2:30 am
Location: South Central Mitten

Re: Acceleration issue

Postby MiGhost » Tue Sep 17, 2013 5:37 pm

If the seafoam/fuel mixture flowed through the holes in the emulsion tube that are plugged it might work. But that is not the way the slow jets work.

Seafoam will only clean the main internal passage of the jet. Not the emusion tube holes that get plugged which allow the air to flow inward to mix with the fuel in the main passage of the jet.

This will give a faulty air/fuel mixture on both the slow jet, and idle mixture screw, and this is what causes the hesitation, and stumbling. You will never get the carbs dialed in correctly while the slow idle curcuit is fouled/dirty.

Once you pull the slow/idle/pilot jets, and find out how easy it actually is to clean them properly. You will wonder why there is so much fuss about not doing it.

Ghost
~ Ghost

PD50 Carb Info
82 CB650SC Brake Caliper Info

Stable
Wayward Son:1980 CB650C, RestoMod Period Custom Touring
Bad Moon: 1983 XJ750MK Midnight Maxim, Full Dress Tour Deluxe w/ X-1 Fairing
Nemesis: 1983 XJ750K Maxim, Old School style chopper

User avatar
Rocket Man
Posts: 27
Joined: Sun Aug 11, 2013 11:17 am

Re: Acceleration issue

Postby Rocket Man » Tue Sep 17, 2013 8:42 pm

I have never dealt with a pressed in jet before, but it sounds straight forward enough. This will be strike 3 for removing the carbs. Must be getting good at it by now. :roll:

MiGhost
Posts: 379
Joined: Tue Jun 07, 2011 2:30 am
Location: South Central Mitten

Re: Acceleration issue

Postby MiGhost » Fri Sep 20, 2013 5:30 pm

Rocket Man wrote:This will be strike 3 for removing the carbs. Must be getting good at it by now. :roll:


Only 3 times! Your just starting to get the hang of it!!

The pressed in slow jets will pull out easily with pliers, and a slight twisting motion. You can use a rag to keep from scratching the brass while doing so.

Ghost
~ Ghost

PD50 Carb Info
82 CB650SC Brake Caliper Info

Stable
Wayward Son:1980 CB650C, RestoMod Period Custom Touring
Bad Moon: 1983 XJ750MK Midnight Maxim, Full Dress Tour Deluxe w/ X-1 Fairing
Nemesis: 1983 XJ750K Maxim, Old School style chopper

User avatar
Rocket Man
Posts: 27
Joined: Sun Aug 11, 2013 11:17 am

Re: Acceleration issue

Postby Rocket Man » Sat Sep 21, 2013 6:15 pm

The jets all look incredibly clean except for some wear/erosion on the main jets. You can not read the 105 on 2 of them. The accelerator pump gap may have been a bit excessive so I took it to zero gap. I'm a bit hesitant to put it all back together yet because there may still be a hole plugged somewhere.

I just found out that stock mains are 90. Could that tend to flood and bog things during acceleration? The air cleaner is stock.
Attachments
mains1.JPG
mains1.JPG (62.22 KiB) Viewed 10347 times
pilots.jpg
pilots.jpg (90.11 KiB) Viewed 10347 times
pilot.jpg
pilot.jpg (50.41 KiB) Viewed 10347 times

usbear
Posts: 361
Joined: Sun Mar 10, 2013 9:55 pm

Re: Acceleration issue

Postby usbear » Sun Sep 22, 2013 12:40 am

Yes, that is too much fuel in stock form!

*Get NEW Jets 90 and 95 ( not used ones)
do not use worn old jets here. You can use old 90 jets and drill them out to the next step with a Drill index (set of drill bits) in the correct size range.
If you drill, make sure to sand the bottom on a flat surface like glass to remove all the stake marks and burrs from the bottom of the jet.
Mark your Jets somehow to indicate you have enlarged them.(engrave or something. Keep good notes.

*Start with 90 and work your way up.


1)where do you live? altitude and climate?
2)do you have stock 4 into 4 exhaust system? Are all exhaust pipes hot when running?
3) SET THE AIR GAP ON THE PULSE GENERATOR. Use a brass feeler gauge (non magnetic) and set them tight like the manual says. It is a small gap and they are normally way out.
4) Clean and test your timing advance also while setting the Pulser gap.
5) is top end tight or is it noisy?
6) Make sure you have the correct tubes to mount the main Jets in. Your look different from my 80 cb650C. Check all components inside to make sure they are correct and the same for each Carb. Match them up.

If someone has been in there and they have , then you can not trust anything.

Check it out for yourself.

Good Luck and best wishes.

User avatar
Rocket Man
Posts: 27
Joined: Sun Aug 11, 2013 11:17 am

Re: Acceleration issue

Postby Rocket Man » Sun Sep 22, 2013 10:35 am

Live on the coast in central California. Stock exhaust and temperatures have been off, but I think that was corrected after going through the spark plug wire connections. I need a brass feeler gauge, but tightened up the gap with a regular one. Valves adjusted and top end sounds pretty good. I'm guessing the original owner wanted to go faster at open throttle. Off to look for new jets.
Attachments
photo.jpg
#35 pilots and 105 mains
photo.jpg (11.64 KiB) Viewed 10333 times

Totaled108
Posts: 65
Joined: Mon Oct 03, 2011 3:45 am

Re: Acceleration issue

Postby Totaled108 » Wed Oct 23, 2013 6:48 am

I'm going to point this issue to air cut off diaphragms being damaged. I had this issue for a long time, then bypassed the air cut off system, now the hesitation is gone.

When I get a chance to rebuild my spare rack, I'll go with new diaphragms.

User avatar
Rocket Man
Posts: 27
Joined: Sun Aug 11, 2013 11:17 am

Re: Acceleration issue

Postby Rocket Man » Thu Oct 24, 2013 10:35 pm

I have heard that the diaphragms are not really needed and a port can be plugged to avoid problems.

User avatar
Acoustoman
Posts: 21
Joined: Tue Mar 05, 2013 12:27 am
Location: Moreno Valley, CA

Re: Acceleration issue

Postby Acoustoman » Fri Oct 25, 2013 10:10 pm

If you're talking about the vacuum petcock, it can definitely be removed. Just plug the vacuum line, or replace the brass fitting (on the carb) with an appropriate screw and washer so there are no vacuum leaks, and remember to turn off your fuel petcock when parked. :wink: BTW, nice pic of the main and slow jets above! Did you pull the slow jets out with a screw or something? When I pulled/cleaned mine, it did wonders for my acceleration.
I currently have: 1981 CB650C, 1973 CB500K; Next project: 1973 CB750K :-j

User avatar
Rocket Man
Posts: 27
Joined: Sun Aug 11, 2013 11:17 am

Re: Acceleration issue

Postby Rocket Man » Sat Oct 26, 2013 11:45 am

These carbs are not the CV type. 1980 cb650c with 7000 miles on it. I like the idea of jamming a toothpick in the carb port to the air cut off valve, especially if I don't have to remove the carbs. I still suspect a spark issue from #4 coil, so I still have some testing to do. Not to mention a plug check. Compression was good :mrgreen:
As for the pilot jets, I used a vice grip with a rag. It took a little wiggling and then a straight pull.

MiGhost
Posts: 379
Joined: Tue Jun 07, 2011 2:30 am
Location: South Central Mitten

Re: Acceleration issue

Postby MiGhost » Sat Oct 26, 2013 8:46 pm

Rocket Man wrote:I like the idea of jamming a toothpick in the carb port to the air cut off valve, especially if I don't have to remove the carbs.


Jamming a toothpick into the air port for the cut off valve is not the correct way to disable its function. It does nothing to eliminate the vacuum source that activates the curcuit. If the diaphrams are bad also. It creates a vacuum leak that affects the slow/pilot curcuit causing the hesitation off idle.

To disable the cut off curcuit correctly you need to plug the vacuum source port in the top left part of the diaphram chamber. If you also plug the passage to the slow/pilot curcuit in the bottom of the chamber. You can eliminate the diaphram.

Ghost
~ Ghost

PD50 Carb Info
82 CB650SC Brake Caliper Info

Stable
Wayward Son:1980 CB650C, RestoMod Period Custom Touring
Bad Moon: 1983 XJ750MK Midnight Maxim, Full Dress Tour Deluxe w/ X-1 Fairing
Nemesis: 1983 XJ750K Maxim, Old School style chopper

User avatar
Rocket Man
Posts: 27
Joined: Sun Aug 11, 2013 11:17 am

Re: Acceleration issue

Postby Rocket Man » Sun Nov 24, 2013 11:01 pm

So I did something similar anyway and got lucky. My diaphragms must be good in the air cut-off valves. Most likely a plugged hole, but it runs great now!! :D :D :D :D
Attachments
image1.jpg
Screws at 8 o'clock
image1.jpg (59.75 KiB) Viewed 10132 times


Return to “Technical Discussion”

Who is online

Users browsing this forum: Google [Bot] and 46 guests