[jordanbailey]

Hey guys,

I recently picked up a '71 CB350 and am in the baby stages of doing some overhaul on it. For the most part, it runs well, kicks over every time and rides relatively smoothly. I've noticed 2 problem areas and was hoping to get some input:

1) One of my exhausts is running hotter than the other. They're both hot, but one is obviously hotter than the other.
2) While riding, the cooler cylinder side is also producing some intermittent popping/backfiring.

Based on those two issues, what makes sense in terms of fully evaluating and fixing the issue(s)? My first thought was that it's a carb sync problem, but I thought it was worth bringing up here to see if any of you have had similar issues and had any suggestions on what steps to take.

Thanks in advance for any and all input or help. Cheers.

 

[J-T]

Likely a combination. Could be mixture adjustment, carb sync, plugs, points, valve adjustment, etc.

Here's some information to help get you started:

http://www.hondatwins.net/forums/67...n/16105-basic-checklist-new-you-old-bike.html

 

[Yendor]

I'd bet on timing but the Checklist posted in the link above will prove out the root casue.

 

[outobie]

if one exhaust is cooler than the other and you are getting backfiring then that cylinder is most likely cutting in and out...not firing consistently.

 

[jordanbailey]

Thanks for the feedback, all of you. I definitely plan on running through Honda's standard issue checklist, straight out of the repair manual. The main reason I posted was to see if anyone had any insight into this combination of issues, and whether their simultaneous presence indicated anything more specific.

Yendor is leaning towards timing. Outobie, you think its a misfiring issue. Can either of you explain why you believe that specific area is the root cause? I'm just curious as I try to evaluate the problem, your insights are very helpful.

Thanks!

 

[outobie]

one pipe noticeably cooler than the other typically means that the cooler pipe is firing intermittently (or not at all)...this is closely related to timing but in addition to dynamically testing timing you should also check the plugs, plug caps, plugs and trim back 1/4" of the high tension lead and reattach the cap) also make sure the points lead isn't shorting out against the points cover and the condenser is good.

these things are good maintenance things to do anyhow so even if not your immediate problem would need to be done sooner rather than later anyhow

 

[Yendor]

My thought on timing is:

These set the tiimng rather different by Gapping The LEFT Side & Setting that side via rotating the Breaker Plate either Static or Dynamic(with a Strobe Light).

Then setting the timing for the Right via Adjusting the Gap for that side and getting it timed either Static or Dynamic(with a Strobe Light).

Dynamic with a Strobe light is ALWAYS BEST Pratice when available. Static will do in a Pinch.

It's easy when doing this to forget to go back and CHECK the Gap for the Right side to be sure it is still witihin the .012" - .016" Range. And If not re-set the Gap on the Left Side by .001" one way or the other and start over. (.001" equal approx 7-10 Deg in timing)

The Points Charge the Coils when they are closed and the ocils fire when the Points Open.

Open too soon the coils don't charge enough, closed too long can burn them up.

 

[jordanbailey]

Awesome, all noted. Thanks again for the assistance and input!

 

[jordanbailey]

I went back and did some further examination. I did a compression test and got 105 left and 130 right. The gauge I used was pretty ancient, however, so I'm gonna retest to make sure those numbers are accurate.

It looks like I'm running on 1 cylinder (the right one). Once the engine is running I can pull the left cap assembly off of the left plug and it continues to run as though nothing has changed, which was baffling to me. The cap assembly gave me a good zap so I know its sending spark to the plug. And I swapped the plugs from left to right to make sure the left plug wasn't the issue.

Given that the compression for both is well below spec, I'm guessing I need a top end job, is that accurate?

 

[fxray]

Pulling the high tension lead from the spark plug while the engine is running, or pulling the lead and then running the engine can destroy the coil. If you want to do this, snap a spare sparkplug into the end of the wire, and make sure the plug is grounded against the head. Then run the engine (don't try to do this while the engine is running).

Ray

 

[Yendor]

The numbers you posted for Compression say you need to do a rebuild/overhaul on that motor.

HOWEVER:
Was the Choke - OPEN - and the Throttle WIDE OPEN.

If not re-do your compresssion test.

When you say Compression Gauge is acient?
Does it actually work?
How long is the hose between the Cylinder and the Guage? - A long hose will give false readings on these motors as there is just not enough VOLUME to the cylinders to fill the hose and register full pressure. You may want to rent one from a local auto store.

But...
don't give up:

Once you know you have a reliable gauge re-do the test WIDE OPEN (best if carbs are actually removed). Both PLUGS OUT so it can crank as fast as possible.
If it still comes up low add a Teaspoon of OIL to each cylinder and Repeat the test.

If the number get significantly better you have a ring issue. They may be just stuck and can sometimes be freed up by filling the cylinders with Seafoam and letting them sit to breakdown any gunk in them.

Let us know what your results are after re-doing the test with oil.

 

[jordanbailey]

Yep, I understand that. Pulling the lead wasn't actually intentional, the cap is broken and doesn't sit snuggly so it was nudged off while the engine was running. The fact that it kept running after one of the leads was disconnected was the bigger surprise as I didn't think that was possible.

 

[jordanbailey]

Yendor, all I meant is that the gauge is old and I'm not 100% sure that its working correctly. I'm going to try another one to cross check as soon as I'm able and I'll make sure to tick those boxes when doing the test.

Another thing that is puzzling me is, there seems to be no combustion going on in the left cylinder (no gas); i checked the coil yesterday and there is spark coming out of the plug wire, but if there was gas getting to the cylinder, then that plug should have been
soaking wet with (unburned) gas...it wasn't...

Any thoughts on that?

 

[Yendor]

You didn't answer the question about hose lenght?
If the Gauge has a longish hose these small bikes don't have enough volume in the cylinders to fill the hose and register full pressure, you may be getting false readings.

As fas as the Fuel issue - You may not be getting fuel to that side.

The simple check is to remove the Drain Plug from that Carb's Float Bowl and see if anything comes out. Be prepared to replace the Drain Plug Quickly.

In theory you should NOT have to completely remove the Drain Plug. They are drilled Hollow and have a side Hole that comes thru the side of the Plug in the threaded area just behind where either the Brass Washer or O-Ring sits depending on your carb model.

All of this assumes the carbs were cleaned out internally recently, and the Drain Bolt checked and verified as clear, If not there may be enough sediment in the bottom of the bowl to clog up the drain hole that would require a toothpick be pushed through to get flow. All of the Carbs for these bikes I have ever opened up have required that hole in both the Float bowl and the drain plug to be cleared of sediment. If your carb does require a toothpick to be pushed into the drain plug hole to get flow - you need to rebuild/clean them. Because if there is enough sediment inside to prevent flow through the drain, there is more than enough to clog jets.

If no flow, the Float/Needle valve may be stuck or that side may not be feeding from your petcock.

Checking the Petcock is even easier just remove the Fuel Line from that side and place a container under the tip and open the valve.

 

[jordanbailey]

Thanks for all that input, Yendor.

...and, the length of the hose on the gauge I used is a little less than a foot.

 

[Yendor]

Thanks for all that input, Yendor.

...and, the length of the hose on the gauge I used is a little less than a foot.

What is the inside diameter of the hose. With that and a little math we can determine the volume of the hose and how much it will influence the compression figures.