Topic: Honda CB350Four overheating after rebuild

[rollo]

Hi all,
I recently rebuilt a CB350Four engine, new pistons, rings, seals etc. I also rebuilt the carbs. The engine started and ran well. After running the engine for about 30km I noticed that the engine was stalling after running for a while (I'm guessing due to overheating) and leaking oil from the bottom head gasket. Given this I decided to buy another gasket kit and replace the top head gasket and bottom head gasket and investigate further. When I removed the cylinder head I noticed cylinders 1 & 4 had excess carbon build up when compared to cylinders 2 & 3. I also noticed a slight rattling coming predominantly from the left side when the engine came up to temperature. Ignition timing was verified to be correct after the overheating issue. I know its not much to go on and if you have any idea what the issue might be I can provide further info. Thanks

[MauiK3]

Carb/fuel delivery?

[rollo]

1 & 4 seem rich but I thought overheating is a result of a lean mixture

[newday777]

Welcome aboard the forum rollo
What oil did you have in it at the time?

[newday777]

What size pistons and rings did you put in?

[calj737]

1 & 4 seem rich but I thought overheating is a result of a lean mixture

True.

You need to pull the plugs as they will be more revealing than the pistons.

[denward17]

Just curious, how does one know if the engine is over heating?

[robvangulik]

Just curious, how does one know if the engine is over heating?

They hardly ever really do!

[calj737]

The OP stated "he was guessing it was an overheating issue" so I wouldn't jump too much onto that as we what he's described isn't an issue per se, but he does have some engine troubles.

The carbon desposits are likely attributed to a fuel mixture and the poor performance likely to a tuning issue overall. The "rattling" on the left side of the engine could be oil starvation, valves out of spec, exhaust leak, or even carb synch.

As he said, not much to go on. Time to commence the rebuild it correctly and assemble it properly in sequence. Head, torqued, carbs bench synched, valves adjusted, ignition set, start it up after a quick peek at the plugs.

[HondaMan]

Question#1, based on your picture of the cylinders above:
-What pistons and rings did you use, and why is there no crosshatch from honing the cylinders?

Here is the [brutal] truth about the 350F/400F: you cannot simply replace the pistons and rings with new ones, as the new ones will never seat, and will continually lose oil (and run hot in the process).

The piston-to-bore clearance in this engine is a mere 0.0004" (that's not a typo! It is 4 ten thousandths of one inch) up to 0.0008" absolute maximum. If the piston clearance exceeds 0.0010" then it will continuously lose and burn oil.

Here's a second thing you need to know about: all of today's head gaskets are 20% thicker than the OEM head gaskets, so you must increase the thickness of the O-rings for the oil feed ports thru the head gasket by 20% when using today's head gaskets, and if you want back the original compression ratio you must also have a machine shop plane off 0.010" from the top of the cylinders while they are apart. This will restore original compression ratios.