Topic: lean condition, fixed

[oldbob]

I'm using a reproduction exhaust (DSS) on my 400. The four headers slip into the collector, but only #1 has a pinch bolt. 3 and 4 are loose enough to show soot collecting around the joint. Based on spark plug color, those two cylinders also look to be running lean. The connections at the cylinder head are good and have fresh gaskets. The "propane test" does not reveal any vacuum leaks at those carbs. I set the float levels recently.

Does it seem reasonable to attribute the lean condition to the loose-fitting pipes? If so, how can I seal the pipe/collector joints?

Thanks.

Bob

[lucky]

Get the right parts for your motorcycle.

[oldbob]

The stock exhaust for the Honda CB400F has not been available for years. The DSS reproduction part is the closest approximation.

[flybox1]

Does it seem reasonable to attribute the lean condition to the loose-fitting pipes? If so, how can I seal the pipe/collector joints?

There are a few poor exhaust sealant kits you can get from your local auto store.  stuff like JB stick (putty) that hardens (and fails) over time.
best to get a welder in there to seal them up properly. until then, the decreased back pressure will make you lean.

[LesterPiglet]

I've had a few different exhausts on my 550, including no muffler/silencer and none ever changed the combustion process. Just my experience.

[iron_worker]

Exhaust backpressure can and does affect carb tuning requirements. I really don't think a slight exhaust leak would have much effect though.

Why don't 2, 3, and 4 have pinch bolts?

IW

[oldbob]

Why don't 2, 3, and 4 have pinch bolts?

Good question.

[iron_worker]

Why don't you get some? Or are the ears broken off? Threads stripped out?

IW

[oldbob]

I didn't explain it clearly. Only one of the spigots is configured for a pinch bolt. The other three are just straight tubes without splits or collars. Not the best design.

Bob

[HondaMan]

By chance...have you vacuum-balanced the carbs, at, say, 1500 RPM?

[oldbob]

No, I haven't.

I bench-synched them, but I can check them with the manometer.

Bob

[iron_worker]

Oh I see. I was thinking of the pinch bolt at the exhaust spigot like used on the 750s.

I wouldn't think the amount of leakage you would get from here would cause a lean condition. It is a strange design though.

IW

[HondaMan]

No, I haven't.

I bench-synched them, but I can check them with the manometer.

Bob

When you do, watch for the 'adjacent cylinder' issue. That is: if one cylinder has higher vacuum than the next one in the firing order, that next one often has dark plugs.

[oldbob]

I just checked the vacuum using my manometer. Cylinders 1 & 4 were within one inch of antifreeze (not mercury) of each other at 1500 rpm. Cylinders 2 & 3 had slightly more vacuum, which I corrected. Now, all cylinders are within 1" of #4.

I also checked float level with a clear plastic tube. #1, #3, and #4 were at the same level. #2 (of course, because it's the most difficult to access) was 1mm lower.

Looking at the plugs, cylinder #1 is rich, #4 is lean (extremely light discoloration on the insulator), and #2 and #3 are where I want them to be. But, according to what I just saw #1 and #4 have exactly the same float level and almost exactly (1") the same vacuum at 1500 rpm.

Just for S&G, I swapped main jets between #1 and #4. Same result. #4 is still lean.

Still, the motor idles well and responds smoothly to the throttle.

It seems that the only things left are the needles.

Bob

[oldbob]

I pulled the slides for #1 and #4 expecting to see the needles at radically different heights. No, they were exactly the same.

Back to plugs.


Now, it looks as if #2 was rich and then subjected to a lean condition. #3 now appears to be rich. Yesterday, they both had an even tan color. #4 worries me.

What am I missing/forgetting?

Thanks.

Bob

[oldbob]

In my initial post, I said that the "propane test" did not indicate any vacuum leaks. On a hunch, I tested the test. I loosened one of the clamps that secures the intake boot to the #4 carburetor, and slightly peeled back the boot. That should introduce a vacuum leak. However, blowing propane (from an unlit torch) directly at the boot produced no discernable change in how the motor idled. So, the test, or more likely, the way in which I performed the test, did not produce useful data. A vacuum leak or leaks could exist.

I sealed the boot to the intake manifold with RTV and let it cure overnight. This morning's test ride shows the #4 plug to have the desired color. Now, I'll seal the other three boots. Yes, #1 should still be rich, but I'll chase that one some other time.

Bob

[mono]

so the RTV helped?  I'm about to replace my boots with "new" ones (replicas cast in silicone i think), but was considering some extra sealant to create an assured vacuum.   

[bjbuchanan]

Is #4 firing? Could be a wasted spark plug cap or wiring not making contact with the plug cap. A not firing cylinder looks like lean because the plug doesn't actually color because there is no combustion process. Happened to me after I searched high and low for vacuum leaks and everything else

Mono, I have used a small amount of permatex ultra black to seal up the boots. Put the small smear on the part that is getting the boot shoved on. So, on the head flanges and on the carb flanges. A small smear is all it takes so you have no worries of overhang in to the passage. Set the boots in place so they are squared up and straight across and leave them be to cure. Come back later after curing and put your clamps on the head side and then small smear on the carb rack and put the rack on and let cure up. Patience gives the best seal with this stuff, especially with beat up clamps

[oldbob]

so the RTV helped?   

It more than helped. The lean condition is gone. What I don't know is how often I will need to re-seal the manifold/boot joints. But now that the leaks are gone, I need to re-synch the carbs.

BJ: #4 is now a happy cylinder. FWIW, I replaced the plugs, caps, and wires recently.

Bob

[bjbuchanan]

Nice, good job. I didn't read through the thread entirely, just kinda chimed in on what I thought might be going on

The rtv should seal just fine if you let it fully cure before you ran the engine. If you don't let it cure up some of it get "eaten" up by the engine. It magically disappears lol. Also if you really yank on the boots taking the carb rack off and have crappy sealing boots/clamps you will probably have your leak back. Otherwise you will be pretty much good to go

[Jerry Rxman Griffin aka MuthaF'er]

Bob,

My 75 750F uses a packing seal for the 'other' pipes. What about your 400?

Just took a look. Joint, gasket. The original has 2 'loose' pipes and each has this packing/gasket. What has DSS told you. They have a project going rebuilding numerous 400's and should have your solution.

[oldbob]

The reply I received from DSS:

"A little exhaust paste around the pipe will be enough to seal it.The manufacturers tell us that carbon will quickly build up and form its own seal."

I'm not familiar with "exhaust paste" so I'll wait to see if the joints are self-sealing.

Bob

[HondaMan]

so the RTV helped?   

It more than helped. The lean condition is gone. What I don't know is how often I will need to re-seal the manifold/boot joints. But now that the leaks are gone, I need to re-synch the carbs.

BJ: #4 is now a happy cylinder. FWIW, I replaced the plugs, caps, and wires recently.

Bob

I just bought 2 full sets of brand-new 350F/400F boots from Honda! They weren't expensive, either. What a surprise! My Baby Four will be SO happy!