My starter turned rite over normally on the first try , but if the engine doesn’t start on the second atempt it barely turns over like the battery is down , but I checked battery and shows 12.8 volts , but just to be sure I installed a second battery with same effect , currently I have replaced all positive and negative cables with 1/0 new ones replaced solonoid with my spare , no change , what could cause this ?
Perfect situation to do voltage drop tests. Since you didn't mention what kind of vehicle you are working on, it's hard to give specific step by step tests. Start with measuring battery voltage while cranking. It shouldn't go below 8.6 volts. If it does, either the battery is weak or the starter draw is too much. If it stays at or above 9.0 volts, start voltage drop testing. Connect the negative voltmeter lead to the negative battery post. Put the positive lead at the first connection of the battery cable going to the starter. Crank the engine, the meter should not read more than 0.5 volts. If it's higher, the cable is bad. If it is 0.5 volts or lower (while cranking), move the positive lead to the next connection in the circuit and crank the engine again. You are still looking for a reading 0.5 volts or lower. If it is higher, the problem is between the first reading point and the last. Keep doing this unit you have the positive lead at the power going into the starter. If you don't go over 0.5 volts total, check the negative side. You can check the negative side by having the negative lead still on the negative terminal of the battery. Touch the positive lead to the starter case and crank the engine. The voltmeter should read less than 0.1 volts. If it is higher, you have a bad ground. Note that all readings are done with the starter cranking. Also, you need a good battery that won't go below 8.6 volts during cranking.
And also put all new positive cables on also , it is a 1946 ford flathead v8 wit 12 volt positive ground system
How is your starter/ignition switch? Worn/dirty contacts may become a high resistance load after the contacts have been heated up after the first try. Could also be the brushes are worn, commutator may need some attention/cleaned up. Had a starter on my Olds get stubborn. Initially I thought it was ye olde sticking GM starter solenoid(heat soaked). Pulled the starter the grease inside was more tacky/gummy than lubricant. Cleaned it up, re-grease, spins much-mo-betta now.
All the previous posts are good information but, is your starter 12v or 6v? I used 6v starters on my 12v system race cars for years and never had a problem until the starter absolutely wore out.
Flathead starters are easy to take apart to check, they practically fall apart in your hands. Loosen the two long bolts at the front, tilt the starter to the inward side, and pull forward. As you get it loose it will probably start coming apart. Clean everything up and check for worn brushes and bushings, and anything that looks like it has been rubbing. I use my flatty's 6 volt starter on 12 volts for going on 14 years now. Spins nice and hasn't burnt up yet.
Had an issue with a starter that had worn bushings. It worked fine when it was cold, but the armature rubbed the casing when it was hot, shorting it out...
Having your battery load tested is an excellent suggestion. However, checking battery voltage while cranking is a load test. If the battery voltage stays above 8.6 volts while the starter is slowing down, your battery is probably okay. Using an amp clamp around a battery cable would confirm whether your problem is battery or starter related. I think 150 -175 amps would be an acceptable starter draw. But remember, the battery voltage can't drop below 8.6 volts.
9.6 volta is the go/no go load test voltage for a 12 volt battery. At 77 F. If a battery drops close to that just cranking the engine over with the starter, it is toast. A healthy battery would be around ~ 11 volts. An actual carbon pile load tester is 1/2 the battery CCA rating applied for 15 seconds. Can you say "torture test"? A neat trick, you can infer the actual starter current draw by adjusting the carbon pile resistance to the same voltage reading previously indicated during the cranking test and read the amp scale. Starter current draw will he listed in the shop manual.
New cables are wonderful but I have gone out and checked "just got it together and it won't crank" rigs only to find that that brand new ground strap was insulated from the block by four coats of engine paint. Do you have clean bare metal contact area under the ground straps? If the ground strap goes to an alternator bracket does the bracket have clean bare metal contact with the head or block? Other than that it isn't hard to pull most starters apart and check the brushes and bushings or bearings and the rest of the parts and if you run into the "how the hell do I get it back together I or someone else can walk you though it on most American starters.
The starter bolt is just a small thing, not big enough for a battery cable. Just yesterday I was jazzing my nephew as he attempted to start his old DD pickup. He popped the hood and the battery cables were more green and fuzzy than anything else. This kid should know better, he had a wrench in his hand since he could walk. Make sure all connection points of cables and ground straps are cleaned to bare metal, and these spots have a good slathering of dielectric grease to protect them.
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